Merge branch 'urgent.2014.06.23a' of git://git.kernel.org/pub/scm/linux/kernel/git...

[cascardo/linux.git] / kernel / rcu / tree.c

/*
 * Read-Copy Update mechanism for mutual exclusion
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, you can access it online at
 * http://www.gnu.org/licenses/gpl-2.0.html.
 *
 * Copyright IBM Corporation, 2008
 *
 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 *          Manfred Spraul <manfred@colorfullife.com>
 *          Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical version
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *      Documentation/RCU
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/nmi.h>
#include <linux/atomic.h>
#include <linux/bitops.h>
#include <linux/export.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/module.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/time.h>
#include <linux/kernel_stat.h>
#include <linux/wait.h>
#include <linux/kthread.h>
#include <linux/prefetch.h>
#include <linux/delay.h>
#include <linux/stop_machine.h>
#include <linux/random.h>
#include <linux/ftrace_event.h>
#include <linux/suspend.h>

#include "tree.h"
#include "rcu.h"

MODULE_ALIAS("rcutree");
#ifdef MODULE_PARAM_PREFIX
#undef MODULE_PARAM_PREFIX
#endif
#define MODULE_PARAM_PREFIX "rcutree."

/* Data structures. */

static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];

/*
 * In order to export the rcu_state name to the tracing tools, it
 * needs to be added in the __tracepoint_string section.
 * This requires defining a separate variable tp_<sname>_varname
 * that points to the string being used, and this will allow
 * the tracing userspace tools to be able to decipher the string
 * address to the matching string.
 */
#define RCU_STATE_INITIALIZER(sname, sabbr, cr) \
static char sname##_varname[] = #sname; \
static const char *tp_##sname##_varname __used __tracepoint_string = sname##_varname; \
struct rcu_state sname##_state = { \
        .level = { &sname##_state.node[0] }, \
        .call = cr, \
        .fqs_state = RCU_GP_IDLE, \
        .gpnum = 0UL - 300UL, \
        .completed = 0UL - 300UL, \
        .orphan_lock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.orphan_lock), \
        .orphan_nxttail = &sname##_state.orphan_nxtlist, \
        .orphan_donetail = &sname##_state.orphan_donelist, \
        .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
        .onoff_mutex = __MUTEX_INITIALIZER(sname##_state.onoff_mutex), \
        .name = sname##_varname, \
        .abbr = sabbr, \
}; \
DEFINE_PER_CPU(struct rcu_data, sname##_data)

RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);

static struct rcu_state *rcu_state_p;
LIST_HEAD(rcu_struct_flavors);

/* Increase (but not decrease) the CONFIG_RCU_FANOUT_LEAF at boot time. */
static int rcu_fanout_leaf = CONFIG_RCU_FANOUT_LEAF;
module_param(rcu_fanout_leaf, int, 0444);
int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
static int num_rcu_lvl[] = {  /* Number of rcu_nodes at specified level. */
        NUM_RCU_LVL_0,
        NUM_RCU_LVL_1,
        NUM_RCU_LVL_2,
        NUM_RCU_LVL_3,
        NUM_RCU_LVL_4,
};
int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */

/*
 * The rcu_scheduler_active variable transitions from zero to one just
 * before the first task is spawned.  So when this variable is zero, RCU
 * can assume that there is but one task, allowing RCU to (for example)
 * optimize synchronize_sched() to a simple barrier().  When this variable
 * is one, RCU must actually do all the hard work required to detect real
 * grace periods.  This variable is also used to suppress boot-time false
 * positives from lockdep-RCU error checking.
 */
int rcu_scheduler_active __read_mostly;
EXPORT_SYMBOL_GPL(rcu_scheduler_active);

/*
 * The rcu_scheduler_fully_active variable transitions from zero to one
 * during the early_initcall() processing, which is after the scheduler
 * is capable of creating new tasks.  So RCU processing (for example,
 * creating tasks for RCU priority boosting) must be delayed until after
 * rcu_scheduler_fully_active transitions from zero to one.  We also
 * currently delay invocation of any RCU callbacks until after this point.
 *
 * It might later prove better for people registering RCU callbacks during
 * early boot to take responsibility for these callbacks, but one step at
 * a time.
 */
static int rcu_scheduler_fully_active __read_mostly;

#ifdef CONFIG_RCU_BOOST

/*
 * Control variables for per-CPU and per-rcu_node kthreads.  These
 * handle all flavors of RCU.
 */
static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task);
DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
DEFINE_PER_CPU(char, rcu_cpu_has_work);

#endif /* #ifdef CONFIG_RCU_BOOST */

static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);
static void invoke_rcu_core(void);
static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);

/*
 * Track the rcutorture test sequence number and the update version
 * number within a given test.  The rcutorture_testseq is incremented
 * on every rcutorture module load and unload, so has an odd value
 * when a test is running.  The rcutorture_vernum is set to zero
 * when rcutorture starts and is incremented on each rcutorture update.
 * These variables enable correlating rcutorture output with the
 * RCU tracing information.
 */
unsigned long rcutorture_testseq;
unsigned long rcutorture_vernum;

/*
 * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
 * permit this function to be invoked without holding the root rcu_node
 * structure's ->lock, but of course results can be subject to change.
 */
static int rcu_gp_in_progress(struct rcu_state *rsp)
{
        return ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum);
}

/*
 * Note a quiescent state.  Because we do not need to know
 * how many quiescent states passed, just if there was at least
 * one since the start of the grace period, this just sets a flag.
 * The caller must have disabled preemption.
 */
void rcu_sched_qs(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_sched_data, cpu);

        if (rdp->passed_quiesce == 0)
                trace_rcu_grace_period(TPS("rcu_sched"), rdp->gpnum, TPS("cpuqs"));
        rdp->passed_quiesce = 1;
}

void rcu_bh_qs(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);

        if (rdp->passed_quiesce == 0)
                trace_rcu_grace_period(TPS("rcu_bh"), rdp->gpnum, TPS("cpuqs"));
        rdp->passed_quiesce = 1;
}

static DEFINE_PER_CPU(int, rcu_sched_qs_mask);

static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
        .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
        .dynticks = ATOMIC_INIT(1),
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
        .dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
        .dynticks_idle = ATOMIC_INIT(1),
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
};

/*
 * Let the RCU core know that this CPU has gone through the scheduler,
 * which is a quiescent state.  This is called when the need for a
 * quiescent state is urgent, so we burn an atomic operation and full
 * memory barriers to let the RCU core know about it, regardless of what
 * this CPU might (or might not) do in the near future.
 *
 * We inform the RCU core by emulating a zero-duration dyntick-idle
 * period, which we in turn do by incrementing the ->dynticks counter
 * by two.
 */
static void rcu_momentary_dyntick_idle(void)
{
        unsigned long flags;
        struct rcu_data *rdp;
        struct rcu_dynticks *rdtp;
        int resched_mask;
        struct rcu_state *rsp;

        local_irq_save(flags);

        /*
         * Yes, we can lose flag-setting operations.  This is OK, because
         * the flag will be set again after some delay.
         */
        resched_mask = raw_cpu_read(rcu_sched_qs_mask);
        raw_cpu_write(rcu_sched_qs_mask, 0);

        /* Find the flavor that needs a quiescent state. */
        for_each_rcu_flavor(rsp) {
                rdp = raw_cpu_ptr(rsp->rda);
                if (!(resched_mask & rsp->flavor_mask))
                        continue;
                smp_mb(); /* rcu_sched_qs_mask before cond_resched_completed. */
                if (ACCESS_ONCE(rdp->mynode->completed) !=
                    ACCESS_ONCE(rdp->cond_resched_completed))
                        continue;

                /*
                 * Pretend to be momentarily idle for the quiescent state.
                 * This allows the grace-period kthread to record the
                 * quiescent state, with no need for this CPU to do anything
                 * further.
                 */
                rdtp = this_cpu_ptr(&rcu_dynticks);
                smp_mb__before_atomic(); /* Earlier stuff before QS. */
                atomic_add(2, &rdtp->dynticks);  /* QS. */
                smp_mb__after_atomic(); /* Later stuff after QS. */
                break;
        }
        local_irq_restore(flags);
}

/*
 * Note a context switch.  This is a quiescent state for RCU-sched,
 * and requires special handling for preemptible RCU.
 * The caller must have disabled preemption.
 */
void rcu_note_context_switch(int cpu)
{
        trace_rcu_utilization(TPS("Start context switch"));
        rcu_sched_qs(cpu);
        rcu_preempt_note_context_switch(cpu);
        if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
                rcu_momentary_dyntick_idle();
        trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);

static long blimit = 10;        /* Maximum callbacks per rcu_do_batch. */
static long qhimark = 10000;    /* If this many pending, ignore blimit. */
static long qlowmark = 100;     /* Once only this many pending, use blimit. */

module_param(blimit, long, 0444);
module_param(qhimark, long, 0444);
module_param(qlowmark, long, 0444);

static ulong jiffies_till_first_fqs = ULONG_MAX;
static ulong jiffies_till_next_fqs = ULONG_MAX;

module_param(jiffies_till_first_fqs, ulong, 0644);
module_param(jiffies_till_next_fqs, ulong, 0644);

/*
 * How long the grace period must be before we start recruiting
 * quiescent-state help from rcu_note_context_switch().
 */
static ulong jiffies_till_sched_qs = HZ / 20;
module_param(jiffies_till_sched_qs, ulong, 0644);

static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
                                  struct rcu_data *rdp);
static void force_qs_rnp(struct rcu_state *rsp,
                         int (*f)(struct rcu_data *rsp, bool *isidle,
                                  unsigned long *maxj),
                         bool *isidle, unsigned long *maxj);
static void force_quiescent_state(struct rcu_state *rsp);
static int rcu_pending(int cpu);

/*
 * Return the number of RCU-sched batches processed thus far for debug & stats.
 */
long rcu_batches_completed_sched(void)
{
        return rcu_sched_state.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_sched);

/*
 * Return the number of RCU BH batches processed thus far for debug & stats.
 */
long rcu_batches_completed_bh(void)
{
        return rcu_bh_state.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);

/*
 * Force a quiescent state.
 */
void rcu_force_quiescent_state(void)
{
        force_quiescent_state(rcu_state_p);
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

/*
 * Force a quiescent state for RCU BH.
 */
void rcu_bh_force_quiescent_state(void)
{
        force_quiescent_state(&rcu_bh_state);
}
EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);

/*
 * Show the state of the grace-period kthreads.
 */
void show_rcu_gp_kthreads(void)
{
        struct rcu_state *rsp;

        for_each_rcu_flavor(rsp) {
                pr_info("%s: wait state: %d ->state: %#lx\n",
                        rsp->name, rsp->gp_state, rsp->gp_kthread->state);
                /* sched_show_task(rsp->gp_kthread); */
        }
}
EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);

/*
 * Record the number of times rcutorture tests have been initiated and
 * terminated.  This information allows the debugfs tracing stats to be
 * correlated to the rcutorture messages, even when the rcutorture module
 * is being repeatedly loaded and unloaded.  In other words, we cannot
 * store this state in rcutorture itself.
 */
void rcutorture_record_test_transition(void)
{
        rcutorture_testseq++;
        rcutorture_vernum = 0;
}
EXPORT_SYMBOL_GPL(rcutorture_record_test_transition);

/*
 * Send along grace-period-related data for rcutorture diagnostics.
 */
void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
                            unsigned long *gpnum, unsigned long *completed)
{
        struct rcu_state *rsp = NULL;

        switch (test_type) {
        case RCU_FLAVOR:
                rsp = rcu_state_p;
                break;
        case RCU_BH_FLAVOR:
                rsp = &rcu_bh_state;
                break;
        case RCU_SCHED_FLAVOR:
                rsp = &rcu_sched_state;
                break;
        default:
                break;
        }
        if (rsp != NULL) {
                *flags = ACCESS_ONCE(rsp->gp_flags);
                *gpnum = ACCESS_ONCE(rsp->gpnum);
                *completed = ACCESS_ONCE(rsp->completed);
                return;
        }
        *flags = 0;
        *gpnum = 0;
        *completed = 0;
}
EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);

/*
 * Record the number of writer passes through the current rcutorture test.
 * This is also used to correlate debugfs tracing stats with the rcutorture
 * messages.
 */
void rcutorture_record_progress(unsigned long vernum)
{
        rcutorture_vernum++;
}
EXPORT_SYMBOL_GPL(rcutorture_record_progress);

/*
 * Force a quiescent state for RCU-sched.
 */
void rcu_sched_force_quiescent_state(void)
{
        force_quiescent_state(&rcu_sched_state);
}
EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);

/*
 * Does the CPU have callbacks ready to be invoked?
 */
static int
cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
{
        return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL] &&
               rdp->nxttail[RCU_DONE_TAIL] != NULL;
}

/*
 * Return the root node of the specified rcu_state structure.
 */
static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
{
        return &rsp->node[0];
}

/*
 * Is there any need for future grace periods?
 * Interrupts must be disabled.  If the caller does not hold the root
 * rnp_node structure's ->lock, the results are advisory only.
 */
static int rcu_future_needs_gp(struct rcu_state *rsp)
{
        struct rcu_node *rnp = rcu_get_root(rsp);
        int idx = (ACCESS_ONCE(rnp->completed) + 1) & 0x1;
        int *fp = &rnp->need_future_gp[idx];

        return ACCESS_ONCE(*fp);
}

/*
 * Does the current CPU require a not-yet-started grace period?
 * The caller must have disabled interrupts to prevent races with
 * normal callback registry.
 */
static int
cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
{
        int i;

        if (rcu_gp_in_progress(rsp))
                return 0;  /* No, a grace period is already in progress. */
        if (rcu_future_needs_gp(rsp))
                return 1;  /* Yes, a no-CBs CPU needs one. */
        if (!rdp->nxttail[RCU_NEXT_TAIL])
                return 0;  /* No, this is a no-CBs (or offline) CPU. */
        if (*rdp->nxttail[RCU_NEXT_READY_TAIL])
                return 1;  /* Yes, this CPU has newly registered callbacks. */
        for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++)
                if (rdp->nxttail[i - 1] != rdp->nxttail[i] &&
                    ULONG_CMP_LT(ACCESS_ONCE(rsp->completed),
                                 rdp->nxtcompleted[i]))
                        return 1;  /* Yes, CBs for future grace period. */
        return 0; /* No grace period needed. */
}

/*
 * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
 *
 * If the new value of the ->dynticks_nesting counter now is zero,
 * we really have entered idle, and must do the appropriate accounting.
 * The caller must have disabled interrupts.
 */
static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
                                bool user)
{
        struct rcu_state *rsp;
        struct rcu_data *rdp;

        trace_rcu_dyntick(TPS("Start"), oldval, rdtp->dynticks_nesting);
        if (!user && !is_idle_task(current)) {
                struct task_struct *idle __maybe_unused =
                        idle_task(smp_processor_id());

                trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
                ftrace_dump(DUMP_ORIG);
                WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
                          current->pid, current->comm,
                          idle->pid, idle->comm); /* must be idle task! */
        }
        for_each_rcu_flavor(rsp) {
                rdp = this_cpu_ptr(rsp->rda);
                do_nocb_deferred_wakeup(rdp);
        }
        rcu_prepare_for_idle(smp_processor_id());
        /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
        smp_mb__before_atomic();  /* See above. */
        atomic_inc(&rdtp->dynticks);
        smp_mb__after_atomic();  /* Force ordering with next sojourn. */
        WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);

        /*
         * It is illegal to enter an extended quiescent state while
         * in an RCU read-side critical section.
         */
        rcu_lockdep_assert(!lock_is_held(&rcu_lock_map),
                           "Illegal idle entry in RCU read-side critical section.");
        rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),
                           "Illegal idle entry in RCU-bh read-side critical section.");
        rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map),
                           "Illegal idle entry in RCU-sched read-side critical section.");
}

/*
 * Enter an RCU extended quiescent state, which can be either the
 * idle loop or adaptive-tickless usermode execution.
 */
static void rcu_eqs_enter(bool user)
{
        long long oldval;
        struct rcu_dynticks *rdtp;

        rdtp = this_cpu_ptr(&rcu_dynticks);
        oldval = rdtp->dynticks_nesting;
        WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
        if ((oldval & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE) {
                rdtp->dynticks_nesting = 0;
                rcu_eqs_enter_common(rdtp, oldval, user);
        } else {
                rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
        }
}

/**
 * rcu_idle_enter - inform RCU that current CPU is entering idle
 *
 * Enter idle mode, in other words, -leave- the mode in which RCU
 * read-side critical sections can occur.  (Though RCU read-side
 * critical sections can occur in irq handlers in idle, a possibility
 * handled by irq_enter() and irq_exit().)
 *
 * We crowbar the ->dynticks_nesting field to zero to allow for
 * the possibility of usermode upcalls having messed up our count
 * of interrupt nesting level during the prior busy period.
 */
void rcu_idle_enter(void)
{
        unsigned long flags;

        local_irq_save(flags);
        rcu_eqs_enter(false);
        rcu_sysidle_enter(this_cpu_ptr(&rcu_dynticks), 0);
        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(rcu_idle_enter);

#ifdef CONFIG_RCU_USER_QS
/**
 * rcu_user_enter - inform RCU that we are resuming userspace.
 *
 * Enter RCU idle mode right before resuming userspace.  No use of RCU
 * is permitted between this call and rcu_user_exit(). This way the
 * CPU doesn't need to maintain the tick for RCU maintenance purposes
 * when the CPU runs in userspace.
 */
void rcu_user_enter(void)
{
        rcu_eqs_enter(1);
}
#endif /* CONFIG_RCU_USER_QS */

/**
 * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
 *
 * Exit from an interrupt handler, which might possibly result in entering
 * idle mode, in other words, leaving the mode in which read-side critical
 * sections can occur.
 *
 * This code assumes that the idle loop never does anything that might
 * result in unbalanced calls to irq_enter() and irq_exit().  If your
 * architecture violates this assumption, RCU will give you what you
 * deserve, good and hard.  But very infrequently and irreproducibly.
 *
 * Use things like work queues to work around this limitation.
 *
 * You have been warned.
 */
void rcu_irq_exit(void)
{
        unsigned long flags;
        long long oldval;
        struct rcu_dynticks *rdtp;

        local_irq_save(flags);
        rdtp = this_cpu_ptr(&rcu_dynticks);
        oldval = rdtp->dynticks_nesting;
        rdtp->dynticks_nesting--;
        WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
        if (rdtp->dynticks_nesting)
                trace_rcu_dyntick(TPS("--="), oldval, rdtp->dynticks_nesting);
        else
                rcu_eqs_enter_common(rdtp, oldval, true);
        rcu_sysidle_enter(rdtp, 1);
        local_irq_restore(flags);
}

/*
 * rcu_eqs_exit_common - current CPU moving away from extended quiescent state
 *
 * If the new value of the ->dynticks_nesting counter was previously zero,
 * we really have exited idle, and must do the appropriate accounting.
 * The caller must have disabled interrupts.
 */
static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
                               int user)
{
        smp_mb__before_atomic();  /* Force ordering w/previous sojourn. */
        atomic_inc(&rdtp->dynticks);
        /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
        smp_mb__after_atomic();  /* See above. */
        WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
        rcu_cleanup_after_idle(smp_processor_id());
        trace_rcu_dyntick(TPS("End"), oldval, rdtp->dynticks_nesting);
        if (!user && !is_idle_task(current)) {
                struct task_struct *idle __maybe_unused =
                        idle_task(smp_processor_id());

                trace_rcu_dyntick(TPS("Error on exit: not idle task"),
                                  oldval, rdtp->dynticks_nesting);
                ftrace_dump(DUMP_ORIG);
                WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
                          current->pid, current->comm,
                          idle->pid, idle->comm); /* must be idle task! */
        }
}

/*
 * Exit an RCU extended quiescent state, which can be either the
 * idle loop or adaptive-tickless usermode execution.
 */
static void rcu_eqs_exit(bool user)
{
        struct rcu_dynticks *rdtp;
        long long oldval;

        rdtp = this_cpu_ptr(&rcu_dynticks);
        oldval = rdtp->dynticks_nesting;
        WARN_ON_ONCE(oldval < 0);
        if (oldval & DYNTICK_TASK_NEST_MASK) {
                rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
        } else {
                rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
                rcu_eqs_exit_common(rdtp, oldval, user);
        }
}

/**
 * rcu_idle_exit - inform RCU that current CPU is leaving idle
 *
 * Exit idle mode, in other words, -enter- the mode in which RCU
 * read-side critical sections can occur.
 *
 * We crowbar the ->dynticks_nesting field to DYNTICK_TASK_NEST to
 * allow for the possibility of usermode upcalls messing up our count
 * of interrupt nesting level during the busy period that is just
 * now starting.
 */
void rcu_idle_exit(void)
{
        unsigned long flags;

        local_irq_save(flags);
        rcu_eqs_exit(false);
        rcu_sysidle_exit(this_cpu_ptr(&rcu_dynticks), 0);
        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(rcu_idle_exit);

#ifdef CONFIG_RCU_USER_QS
/**
 * rcu_user_exit - inform RCU that we are exiting userspace.
 *
 * Exit RCU idle mode while entering the kernel because it can
 * run a RCU read side critical section anytime.
 */
void rcu_user_exit(void)
{
        rcu_eqs_exit(1);
}
#endif /* CONFIG_RCU_USER_QS */

/**
 * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
 *
 * Enter an interrupt handler, which might possibly result in exiting
 * idle mode, in other words, entering the mode in which read-side critical
 * sections can occur.
 *
 * Note that the Linux kernel is fully capable of entering an interrupt
 * handler that it never exits, for example when doing upcalls to
 * user mode!  This code assumes that the idle loop never does upcalls to
 * user mode.  If your architecture does do upcalls from the idle loop (or
 * does anything else that results in unbalanced calls to the irq_enter()
 * and irq_exit() functions), RCU will give you what you deserve, good
 * and hard.  But very infrequently and irreproducibly.
 *
 * Use things like work queues to work around this limitation.
 *
 * You have been warned.
 */
void rcu_irq_enter(void)
{
        unsigned long flags;
        struct rcu_dynticks *rdtp;
        long long oldval;

        local_irq_save(flags);
        rdtp = this_cpu_ptr(&rcu_dynticks);
        oldval = rdtp->dynticks_nesting;
        rdtp->dynticks_nesting++;
        WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
        if (oldval)
                trace_rcu_dyntick(TPS("++="), oldval, rdtp->dynticks_nesting);
        else
                rcu_eqs_exit_common(rdtp, oldval, true);
        rcu_sysidle_exit(rdtp, 1);
        local_irq_restore(flags);
}

/**
 * rcu_nmi_enter - inform RCU of entry to NMI context
 *
 * If the CPU was idle with dynamic ticks active, and there is no
 * irq handler running, this updates rdtp->dynticks_nmi to let the
 * RCU grace-period handling know that the CPU is active.
 */
void rcu_nmi_enter(void)
{
        struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);

        if (rdtp->dynticks_nmi_nesting == 0 &&
            (atomic_read(&rdtp->dynticks) & 0x1))
                return;
        rdtp->dynticks_nmi_nesting++;
        smp_mb__before_atomic();  /* Force delay from prior write. */
        atomic_inc(&rdtp->dynticks);
        /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
        smp_mb__after_atomic();  /* See above. */
        WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
}

/**
 * rcu_nmi_exit - inform RCU of exit from NMI context
 *
 * If the CPU was idle with dynamic ticks active, and there is no
 * irq handler running, this updates rdtp->dynticks_nmi to let the
 * RCU grace-period handling know that the CPU is no longer active.
 */
void rcu_nmi_exit(void)
{
        struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);

        if (rdtp->dynticks_nmi_nesting == 0 ||
            --rdtp->dynticks_nmi_nesting != 0)
                return;
        /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
        smp_mb__before_atomic();  /* See above. */
        atomic_inc(&rdtp->dynticks);
        smp_mb__after_atomic();  /* Force delay to next write. */
        WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
}

/**
 * __rcu_is_watching - are RCU read-side critical sections safe?
 *
 * Return true if RCU is watching the running CPU, which means that
 * this CPU can safely enter RCU read-side critical sections.  Unlike
 * rcu_is_watching(), the caller of __rcu_is_watching() must have at
 * least disabled preemption.
 */
bool notrace __rcu_is_watching(void)
{
        return atomic_read(this_cpu_ptr(&rcu_dynticks.dynticks)) & 0x1;
}

/**
 * rcu_is_watching - see if RCU thinks that the current CPU is idle
 *
 * If the current CPU is in its idle loop and is neither in an interrupt
 * or NMI handler, return true.
 */
bool notrace rcu_is_watching(void)
{
        int ret;

        preempt_disable();
        ret = __rcu_is_watching();
        preempt_enable();
        return ret;
}
EXPORT_SYMBOL_GPL(rcu_is_watching);

#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)

/*
 * Is the current CPU online?  Disable preemption to avoid false positives
 * that could otherwise happen due to the current CPU number being sampled,
 * this task being preempted, its old CPU being taken offline, resuming
 * on some other CPU, then determining that its old CPU is now offline.
 * It is OK to use RCU on an offline processor during initial boot, hence
 * the check for rcu_scheduler_fully_active.  Note also that it is OK
 * for a CPU coming online to use RCU for one jiffy prior to marking itself
 * online in the cpu_online_mask.  Similarly, it is OK for a CPU going
 * offline to continue to use RCU for one jiffy after marking itself
 * offline in the cpu_online_mask.  This leniency is necessary given the
 * non-atomic nature of the online and offline processing, for example,
 * the fact that a CPU enters the scheduler after completing the CPU_DYING
 * notifiers.
 *
 * This is also why RCU internally marks CPUs online during the
 * CPU_UP_PREPARE phase and offline during the CPU_DEAD phase.
 *
 * Disable checking if in an NMI handler because we cannot safely report
 * errors from NMI handlers anyway.
 */
bool rcu_lockdep_current_cpu_online(void)
{
        struct rcu_data *rdp;
        struct rcu_node *rnp;
        bool ret;

        if (in_nmi())
                return true;
        preempt_disable();
        rdp = this_cpu_ptr(&rcu_sched_data);
        rnp = rdp->mynode;
        ret = (rdp->grpmask & rnp->qsmaskinit) ||
              !rcu_scheduler_fully_active;
        preempt_enable();
        return ret;
}
EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);

#endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */

/**
 * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
 *
 * If the current CPU is idle or running at a first-level (not nested)
 * interrupt from idle, return true.  The caller must have at least
 * disabled preemption.
 */
static int rcu_is_cpu_rrupt_from_idle(void)
{
        return __this_cpu_read(rcu_dynticks.dynticks_nesting) <= 1;
}

/*
 * Snapshot the specified CPU's dynticks counter so that we can later
 * credit them with an implicit quiescent state.  Return 1 if this CPU
 * is in dynticks idle mode, which is an extended quiescent state.
 */
static int dyntick_save_progress_counter(struct rcu_data *rdp,
                                         bool *isidle, unsigned long *maxj)
{
        rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
        rcu_sysidle_check_cpu(rdp, isidle, maxj);
        if ((rdp->dynticks_snap & 0x1) == 0) {
                trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
                return 1;
        } else {
                return 0;
        }
}

/*
 * This function really isn't for public consumption, but RCU is special in
 * that context switches can allow the state machine to make progress.
 */
extern void resched_cpu(int cpu);

/*
 * Return true if the specified CPU has passed through a quiescent
 * state by virtue of being in or having passed through an dynticks
 * idle state since the last call to dyntick_save_progress_counter()
 * for this same CPU, or by virtue of having been offline.
 */
static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
                                    bool *isidle, unsigned long *maxj)
{
        unsigned int curr;
        int *rcrmp;
        unsigned int snap;

        curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
        snap = (unsigned int)rdp->dynticks_snap;

        /*
         * If the CPU passed through or entered a dynticks idle phase with
         * no active irq/NMI handlers, then we can safely pretend that the CPU
         * already acknowledged the request to pass through a quiescent
         * state.  Either way, that CPU cannot possibly be in an RCU
         * read-side critical section that started before the beginning
         * of the current RCU grace period.
         */
        if ((curr & 0x1) == 0 || UINT_CMP_GE(curr, snap + 2)) {
                trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
                rdp->dynticks_fqs++;
                return 1;
        }

        /*
         * Check for the CPU being offline, but only if the grace period
         * is old enough.  We don't need to worry about the CPU changing
         * state: If we see it offline even once, it has been through a
         * quiescent state.
         *
         * The reason for insisting that the grace period be at least
         * one jiffy old is that CPUs that are not quite online and that
         * have just gone offline can still execute RCU read-side critical
         * sections.
         */
        if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
                return 0;  /* Grace period is not old enough. */
        barrier();
        if (cpu_is_offline(rdp->cpu)) {
                trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("ofl"));
                rdp->offline_fqs++;
                return 1;
        }

        /*
         * A CPU running for an extended time within the kernel can
         * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
         * even context-switching back and forth between a pair of
         * in-kernel CPU-bound tasks cannot advance grace periods.
         * So if the grace period is old enough, make the CPU pay attention.
         * Note that the unsynchronized assignments to the per-CPU
         * rcu_sched_qs_mask variable are safe.  Yes, setting of
         * bits can be lost, but they will be set again on the next
         * force-quiescent-state pass.  So lost bit sets do not result
         * in incorrect behavior, merely in a grace period lasting
         * a few jiffies longer than it might otherwise.  Because
         * there are at most four threads involved, and because the
         * updates are only once every few jiffies, the probability of
         * lossage (and thus of slight grace-period extension) is
         * quite low.
         *
         * Note that if the jiffies_till_sched_qs boot/sysfs parameter
         * is set too high, we override with half of the RCU CPU stall
         * warning delay.
         */
        rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
        if (ULONG_CMP_GE(jiffies,
                         rdp->rsp->gp_start + jiffies_till_sched_qs) ||
            ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
                if (!(ACCESS_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
                        ACCESS_ONCE(rdp->cond_resched_completed) =
                                ACCESS_ONCE(rdp->mynode->completed);
                        smp_mb(); /* ->cond_resched_completed before *rcrmp. */
                        ACCESS_ONCE(*rcrmp) =
                                ACCESS_ONCE(*rcrmp) + rdp->rsp->flavor_mask;
                        resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
                        rdp->rsp->jiffies_resched += 5; /* Enable beating. */
                } else if (ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
                        /* Time to beat on that CPU again! */
                        resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
                        rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
                }
        }

        return 0;
}

static void record_gp_stall_check_time(struct rcu_state *rsp)
{
        unsigned long j = jiffies;
        unsigned long j1;

        rsp->gp_start = j;
        smp_wmb(); /* Record start time before stall time. */
        j1 = rcu_jiffies_till_stall_check();
        ACCESS_ONCE(rsp->jiffies_stall) = j + j1;
        rsp->jiffies_resched = j + j1 / 2;
}

/*
 * Dump stacks of all tasks running on stalled CPUs.  This is a fallback
 * for architectures that do not implement trigger_all_cpu_backtrace().
 * The NMI-triggered stack traces are more accurate because they are
 * printed by the target CPU.
 */
static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
{
        int cpu;
        unsigned long flags;
        struct rcu_node *rnp;

        rcu_for_each_leaf_node(rsp, rnp) {
                raw_spin_lock_irqsave(&rnp->lock, flags);
                if (rnp->qsmask != 0) {
                        for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                                if (rnp->qsmask & (1UL << cpu))
                                        dump_cpu_task(rnp->grplo + cpu);
                }
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }
}

static void print_other_cpu_stall(struct rcu_state *rsp)
{
        int cpu;
        long delta;
        unsigned long flags;
        int ndetected = 0;
        struct rcu_node *rnp = rcu_get_root(rsp);
        long totqlen = 0;

        /* Only let one CPU complain about others per time interval. */

        raw_spin_lock_irqsave(&rnp->lock, flags);
        delta = jiffies - ACCESS_ONCE(rsp->jiffies_stall);
        if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }
        ACCESS_ONCE(rsp->jiffies_stall) = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
        raw_spin_unlock_irqrestore(&rnp->lock, flags);

        /*
         * OK, time to rat on our buddy...
         * See Documentation/RCU/stallwarn.txt for info on how to debug
         * RCU CPU stall warnings.
         */
        pr_err("INFO: %s detected stalls on CPUs/tasks:",
               rsp->name);
        print_cpu_stall_info_begin();
        rcu_for_each_leaf_node(rsp, rnp) {
                raw_spin_lock_irqsave(&rnp->lock, flags);
                ndetected += rcu_print_task_stall(rnp);
                if (rnp->qsmask != 0) {
                        for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
                                if (rnp->qsmask & (1UL << cpu)) {
                                        print_cpu_stall_info(rsp,
                                                             rnp->grplo + cpu);
                                        ndetected++;
                                }
                }
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }

        /*
         * Now rat on any tasks that got kicked up to the root rcu_node
         * due to CPU offlining.
         */
        rnp = rcu_get_root(rsp);
        raw_spin_lock_irqsave(&rnp->lock, flags);
        ndetected += rcu_print_task_stall(rnp);
        raw_spin_unlock_irqrestore(&rnp->lock, flags);

        print_cpu_stall_info_end();
        for_each_possible_cpu(cpu)
                totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
        pr_cont("(detected by %d, t=%ld jiffies, g=%ld, c=%ld, q=%lu)\n",
               smp_processor_id(), (long)(jiffies - rsp->gp_start),
               (long)rsp->gpnum, (long)rsp->completed, totqlen);
        if (ndetected == 0)
                pr_err("INFO: Stall ended before state dump start\n");
        else if (!trigger_all_cpu_backtrace())
                rcu_dump_cpu_stacks(rsp);

        /* Complain about tasks blocking the grace period. */

        rcu_print_detail_task_stall(rsp);

        force_quiescent_state(rsp);  /* Kick them all. */
}

static void print_cpu_stall(struct rcu_state *rsp)
{
        int cpu;
        unsigned long flags;
        struct rcu_node *rnp = rcu_get_root(rsp);
        long totqlen = 0;

        /*
         * OK, time to rat on ourselves...
         * See Documentation/RCU/stallwarn.txt for info on how to debug
         * RCU CPU stall warnings.
         */
        pr_err("INFO: %s self-detected stall on CPU", rsp->name);
        print_cpu_stall_info_begin();
        print_cpu_stall_info(rsp, smp_processor_id());
        print_cpu_stall_info_end();
        for_each_possible_cpu(cpu)
                totqlen += per_cpu_ptr(rsp->rda, cpu)->qlen;
        pr_cont(" (t=%lu jiffies g=%ld c=%ld q=%lu)\n",
                jiffies - rsp->gp_start,
                (long)rsp->gpnum, (long)rsp->completed, totqlen);
        if (!trigger_all_cpu_backtrace())
                dump_stack();

        raw_spin_lock_irqsave(&rnp->lock, flags);
        if (ULONG_CMP_GE(jiffies, ACCESS_ONCE(rsp->jiffies_stall)))
                ACCESS_ONCE(rsp->jiffies_stall) = jiffies +
                                     3 * rcu_jiffies_till_stall_check() + 3;
        raw_spin_unlock_irqrestore(&rnp->lock, flags);

        /*
         * Attempt to revive the RCU machinery by forcing a context switch.
         *
         * A context switch would normally allow the RCU state machine to make
         * progress and it could be we're stuck in kernel space without context
         * switches for an entirely unreasonable amount of time.
         */
        resched_cpu(smp_processor_id());
}

static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long completed;
        unsigned long gpnum;
        unsigned long gps;
        unsigned long j;
        unsigned long js;
        struct rcu_node *rnp;

        if (rcu_cpu_stall_suppress || !rcu_gp_in_progress(rsp))
                return;
        j = jiffies;

        /*
         * Lots of memory barriers to reject false positives.
         *
         * The idea is to pick up rsp->gpnum, then rsp->jiffies_stall,
         * then rsp->gp_start, and finally rsp->completed.  These values
         * are updated in the opposite order with memory barriers (or
         * equivalent) during grace-period initialization and cleanup.
         * Now, a false positive can occur if we get an new value of
         * rsp->gp_start and a old value of rsp->jiffies_stall.  But given
         * the memory barriers, the only way that this can happen is if one
         * grace period ends and another starts between these two fetches.
         * Detect this by comparing rsp->completed with the previous fetch
         * from rsp->gpnum.
         *
         * Given this check, comparisons of jiffies, rsp->jiffies_stall,
         * and rsp->gp_start suffice to forestall false positives.
         */
        gpnum = ACCESS_ONCE(rsp->gpnum);
        smp_rmb(); /* Pick up ->gpnum first... */
        js = ACCESS_ONCE(rsp->jiffies_stall);
        smp_rmb(); /* ...then ->jiffies_stall before the rest... */
        gps = ACCESS_ONCE(rsp->gp_start);
        smp_rmb(); /* ...and finally ->gp_start before ->completed. */
        completed = ACCESS_ONCE(rsp->completed);
        if (ULONG_CMP_GE(completed, gpnum) ||
            ULONG_CMP_LT(j, js) ||
            ULONG_CMP_GE(gps, js))
                return; /* No stall or GP completed since entering function. */
        rnp = rdp->mynode;
        if (rcu_gp_in_progress(rsp) &&
            (ACCESS_ONCE(rnp->qsmask) & rdp->grpmask)) {

                /* We haven't checked in, so go dump stack. */
                print_cpu_stall(rsp);

        } else if (rcu_gp_in_progress(rsp) &&
                   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {

                /* They had a few time units to dump stack, so complain. */
                print_other_cpu_stall(rsp);
        }
}

/**
 * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
 *
 * Set the stall-warning timeout way off into the future, thus preventing
 * any RCU CPU stall-warning messages from appearing in the current set of
 * RCU grace periods.
 *
 * The caller must disable hard irqs.
 */
void rcu_cpu_stall_reset(void)
{
        struct rcu_state *rsp;

        for_each_rcu_flavor(rsp)
                ACCESS_ONCE(rsp->jiffies_stall) = jiffies + ULONG_MAX / 2;
}

/*
 * Initialize the specified rcu_data structure's callback list to empty.
 */
static void init_callback_list(struct rcu_data *rdp)
{
        int i;

        if (init_nocb_callback_list(rdp))
                return;
        rdp->nxtlist = NULL;
        for (i = 0; i < RCU_NEXT_SIZE; i++)
                rdp->nxttail[i] = &rdp->nxtlist;
}

/*
 * Determine the value that ->completed will have at the end of the
 * next subsequent grace period.  This is used to tag callbacks so that
 * a CPU can invoke callbacks in a timely fashion even if that CPU has
 * been dyntick-idle for an extended period with callbacks under the
 * influence of RCU_FAST_NO_HZ.
 *
 * The caller must hold rnp->lock with interrupts disabled.
 */
static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
                                       struct rcu_node *rnp)
{
        /*
         * If RCU is idle, we just wait for the next grace period.
         * But we can only be sure that RCU is idle if we are looking
         * at the root rcu_node structure -- otherwise, a new grace
         * period might have started, but just not yet gotten around
         * to initializing the current non-root rcu_node structure.
         */
        if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed)
                return rnp->completed + 1;

        /*
         * Otherwise, wait for a possible partial grace period and
         * then the subsequent full grace period.
         */
        return rnp->completed + 2;
}

/*
 * Trace-event helper function for rcu_start_future_gp() and
 * rcu_nocb_wait_gp().
 */
static void trace_rcu_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
                                unsigned long c, const char *s)
{
        trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
                                      rnp->completed, c, rnp->level,
                                      rnp->grplo, rnp->grphi, s);
}

/*
 * Start some future grace period, as needed to handle newly arrived
 * callbacks.  The required future grace periods are recorded in each
 * rcu_node structure's ->need_future_gp field.  Returns true if there
 * is reason to awaken the grace-period kthread.
 *
 * The caller must hold the specified rcu_node structure's ->lock.
 */
static bool __maybe_unused
rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
                    unsigned long *c_out)
{
        unsigned long c;
        int i;
        bool ret = false;
        struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);

        /*
         * Pick up grace-period number for new callbacks.  If this
         * grace period is already marked as needed, return to the caller.
         */
        c = rcu_cbs_completed(rdp->rsp, rnp);
        trace_rcu_future_gp(rnp, rdp, c, TPS("Startleaf"));
        if (rnp->need_future_gp[c & 0x1]) {
                trace_rcu_future_gp(rnp, rdp, c, TPS("Prestartleaf"));
                goto out;
        }

        /*
         * If either this rcu_node structure or the root rcu_node structure
         * believe that a grace period is in progress, then we must wait
         * for the one following, which is in "c".  Because our request
         * will be noticed at the end of the current grace period, we don't
         * need to explicitly start one.
         */
        if (rnp->gpnum != rnp->completed ||
            ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
                rnp->need_future_gp[c & 0x1]++;
                trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleaf"));
                goto out;
        }

        /*
         * There might be no grace period in progress.  If we don't already
         * hold it, acquire the root rcu_node structure's lock in order to
         * start one (if needed).
         */
        if (rnp != rnp_root) {
                raw_spin_lock(&rnp_root->lock);
                smp_mb__after_unlock_lock();
        }

        /*
         * Get a new grace-period number.  If there really is no grace
         * period in progress, it will be smaller than the one we obtained
         * earlier.  Adjust callbacks as needed.  Note that even no-CBs
         * CPUs have a ->nxtcompleted[] array, so no no-CBs checks needed.
         */
        c = rcu_cbs_completed(rdp->rsp, rnp_root);
        for (i = RCU_DONE_TAIL; i < RCU_NEXT_TAIL; i++)
                if (ULONG_CMP_LT(c, rdp->nxtcompleted[i]))
                        rdp->nxtcompleted[i] = c;

        /*
         * If the needed for the required grace period is already
         * recorded, trace and leave.
         */
        if (rnp_root->need_future_gp[c & 0x1]) {
                trace_rcu_future_gp(rnp, rdp, c, TPS("Prestartedroot"));
                goto unlock_out;
        }

        /* Record the need for the future grace period. */
        rnp_root->need_future_gp[c & 0x1]++;

        /* If a grace period is not already in progress, start one. */
        if (rnp_root->gpnum != rnp_root->completed) {
                trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleafroot"));
        } else {
                trace_rcu_future_gp(rnp, rdp, c, TPS("Startedroot"));
                ret = rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
        }
unlock_out:
        if (rnp != rnp_root)
                raw_spin_unlock(&rnp_root->lock);
out:
        if (c_out != NULL)
                *c_out = c;
        return ret;
}

/*
 * Clean up any old requests for the just-ended grace period.  Also return
 * whether any additional grace periods have been requested.  Also invoke
 * rcu_nocb_gp_cleanup() in order to wake up any no-callbacks kthreads
 * waiting for this grace period to complete.
 */
static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
{
        int c = rnp->completed;
        int needmore;
        struct rcu_data *rdp = this_cpu_ptr(rsp->rda);

        rcu_nocb_gp_cleanup(rsp, rnp);
        rnp->need_future_gp[c & 0x1] = 0;
        needmore = rnp->need_future_gp[(c + 1) & 0x1];
        trace_rcu_future_gp(rnp, rdp, c,
                            needmore ? TPS("CleanupMore") : TPS("Cleanup"));
        return needmore;
}

/*
 * Awaken the grace-period kthread for the specified flavor of RCU.
 * Don't do a self-awaken, and don't bother awakening when there is
 * nothing for the grace-period kthread to do (as in several CPUs
 * raced to awaken, and we lost), and finally don't try to awaken
 * a kthread that has not yet been created.
 */
static void rcu_gp_kthread_wake(struct rcu_state *rsp)
{
        if (current == rsp->gp_kthread ||
            !ACCESS_ONCE(rsp->gp_flags) ||
            !rsp->gp_kthread)
                return;
        wake_up(&rsp->gp_wq);
}

/*
 * If there is room, assign a ->completed number to any callbacks on
 * this CPU that have not already been assigned.  Also accelerate any
 * callbacks that were previously assigned a ->completed number that has
 * since proven to be too conservative, which can happen if callbacks get
 * assigned a ->completed number while RCU is idle, but with reference to
 * a non-root rcu_node structure.  This function is idempotent, so it does
 * not hurt to call it repeatedly.  Returns an flag saying that we should
 * awaken the RCU grace-period kthread.
 *
 * The caller must hold rnp->lock with interrupts disabled.
 */
static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
                               struct rcu_data *rdp)
{
        unsigned long c;
        int i;
        bool ret;

        /* If the CPU has no callbacks, nothing to do. */
        if (!rdp->nxttail[RCU_NEXT_TAIL] || !*rdp->nxttail[RCU_DONE_TAIL])
                return false;

        /*
         * Starting from the sublist containing the callbacks most
         * recently assigned a ->completed number and working down, find the
         * first sublist that is not assignable to an upcoming grace period.
         * Such a sublist has something in it (first two tests) and has
         * a ->completed number assigned that will complete sooner than
         * the ->completed number for newly arrived callbacks (last test).
         *
         * The key point is that any later sublist can be assigned the
         * same ->completed number as the newly arrived callbacks, which
         * means that the callbacks in any of these later sublist can be
         * grouped into a single sublist, whether or not they have already
         * been assigned a ->completed number.
         */
        c = rcu_cbs_completed(rsp, rnp);
        for (i = RCU_NEXT_TAIL - 1; i > RCU_DONE_TAIL; i--)
                if (rdp->nxttail[i] != rdp->nxttail[i - 1] &&
                    !ULONG_CMP_GE(rdp->nxtcompleted[i], c))
                        break;

        /*
         * If there are no sublist for unassigned callbacks, leave.
         * At the same time, advance "i" one sublist, so that "i" will
         * index into the sublist where all the remaining callbacks should
         * be grouped into.
         */
        if (++i >= RCU_NEXT_TAIL)
                return false;

        /*
         * Assign all subsequent callbacks' ->completed number to the next
         * full grace period and group them all in the sublist initially
         * indexed by "i".
         */
        for (; i <= RCU_NEXT_TAIL; i++) {
                rdp->nxttail[i] = rdp->nxttail[RCU_NEXT_TAIL];
                rdp->nxtcompleted[i] = c;
        }
        /* Record any needed additional grace periods. */
        ret = rcu_start_future_gp(rnp, rdp, NULL);

        /* Trace depending on how much we were able to accelerate. */
        if (!*rdp->nxttail[RCU_WAIT_TAIL])
                trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccWaitCB"));
        else
                trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccReadyCB"));
        return ret;
}

/*
 * Move any callbacks whose grace period has completed to the
 * RCU_DONE_TAIL sublist, then compact the remaining sublists and
 * assign ->completed numbers to any callbacks in the RCU_NEXT_TAIL
 * sublist.  This function is idempotent, so it does not hurt to
 * invoke it repeatedly.  As long as it is not invoked -too- often...
 * Returns true if the RCU grace-period kthread needs to be awakened.
 *
 * The caller must hold rnp->lock with interrupts disabled.
 */
static bool rcu_advance_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
                            struct rcu_data *rdp)
{
        int i, j;

        /* If the CPU has no callbacks, nothing to do. */
        if (!rdp->nxttail[RCU_NEXT_TAIL] || !*rdp->nxttail[RCU_DONE_TAIL])
                return false;

        /*
         * Find all callbacks whose ->completed numbers indicate that they
         * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.
         */
        for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
                if (ULONG_CMP_LT(rnp->completed, rdp->nxtcompleted[i]))
                        break;
                rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[i];
        }
        /* Clean up any sublist tail pointers that were misordered above. */
        for (j = RCU_WAIT_TAIL; j < i; j++)
                rdp->nxttail[j] = rdp->nxttail[RCU_DONE_TAIL];

        /* Copy down callbacks to fill in empty sublists. */
        for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++, j++) {
                if (rdp->nxttail[j] == rdp->nxttail[RCU_NEXT_TAIL])
                        break;
                rdp->nxttail[j] = rdp->nxttail[i];
                rdp->nxtcompleted[j] = rdp->nxtcompleted[i];
        }

        /* Classify any remaining callbacks. */
        return rcu_accelerate_cbs(rsp, rnp, rdp);
}

/*
 * Update CPU-local rcu_data state to record the beginnings and ends of
 * grace periods.  The caller must hold the ->lock of the leaf rcu_node
 * structure corresponding to the current CPU, and must have irqs disabled.
 * Returns true if the grace-period kthread needs to be awakened.
 */
static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
                              struct rcu_data *rdp)
{
        bool ret;

        /* Handle the ends of any preceding grace periods first. */
        if (rdp->completed == rnp->completed) {

                /* No grace period end, so just accelerate recent callbacks. */
                ret = rcu_accelerate_cbs(rsp, rnp, rdp);

        } else {

                /* Advance callbacks. */
                ret = rcu_advance_cbs(rsp, rnp, rdp);

                /* Remember that we saw this grace-period completion. */
                rdp->completed = rnp->completed;
                trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuend"));
        }

        if (rdp->gpnum != rnp->gpnum) {
                /*
                 * If the current grace period is waiting for this CPU,
                 * set up to detect a quiescent state, otherwise don't
                 * go looking for one.
                 */
                rdp->gpnum = rnp->gpnum;
                trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpustart"));
                rdp->passed_quiesce = 0;
                rdp->qs_pending = !!(rnp->qsmask & rdp->grpmask);
                zero_cpu_stall_ticks(rdp);
        }
        return ret;
}

static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        bool needwake;
        struct rcu_node *rnp;

        local_irq_save(flags);
        rnp = rdp->mynode;
        if ((rdp->gpnum == ACCESS_ONCE(rnp->gpnum) &&
             rdp->completed == ACCESS_ONCE(rnp->completed)) || /* w/out lock. */
            !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */
                local_irq_restore(flags);
                return;
        }
        smp_mb__after_unlock_lock();
        needwake = __note_gp_changes(rsp, rnp, rdp);
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
        if (needwake)
                rcu_gp_kthread_wake(rsp);
}

/*
 * Initialize a new grace period.  Return 0 if no grace period required.
 */
static int rcu_gp_init(struct rcu_state *rsp)
{
        struct rcu_data *rdp;
        struct rcu_node *rnp = rcu_get_root(rsp);

        rcu_bind_gp_kthread();
        raw_spin_lock_irq(&rnp->lock);
        smp_mb__after_unlock_lock();
        if (!ACCESS_ONCE(rsp->gp_flags)) {
                /* Spurious wakeup, tell caller to go back to sleep.  */
                raw_spin_unlock_irq(&rnp->lock);
                return 0;
        }
        ACCESS_ONCE(rsp->gp_flags) = 0; /* Clear all flags: New grace period. */

        if (WARN_ON_ONCE(rcu_gp_in_progress(rsp))) {
                /*
                 * Grace period already in progress, don't start another.
                 * Not supposed to be able to happen.
                 */
                raw_spin_unlock_irq(&rnp->lock);
                return 0;
        }

        /* Advance to a new grace period and initialize state. */
        record_gp_stall_check_time(rsp);
        /* Record GP times before starting GP, hence smp_store_release(). */
        smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
        trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
        raw_spin_unlock_irq(&rnp->lock);

        /* Exclude any concurrent CPU-hotplug operations. */
        mutex_lock(&rsp->onoff_mutex);
        smp_mb__after_unlock_lock(); /* ->gpnum increment before GP! */

        /*
         * Set the quiescent-state-needed bits in all the rcu_node
         * structures for all currently online CPUs in breadth-first order,
         * starting from the root rcu_node structure, relying on the layout
         * of the tree within the rsp->node[] array.  Note that other CPUs
         * will access only the leaves of the hierarchy, thus seeing that no
         * grace period is in progress, at least until the corresponding
         * leaf node has been initialized.  In addition, we have excluded
         * CPU-hotplug operations.
         *
         * The grace period cannot complete until the initialization
         * process finishes, because this kthread handles both.
         */
        rcu_for_each_node_breadth_first(rsp, rnp) {
                raw_spin_lock_irq(&rnp->lock);
                smp_mb__after_unlock_lock();
                rdp = this_cpu_ptr(rsp->rda);
                rcu_preempt_check_blocked_tasks(rnp);
                rnp->qsmask = rnp->qsmaskinit;
                ACCESS_ONCE(rnp->gpnum) = rsp->gpnum;
                WARN_ON_ONCE(rnp->completed != rsp->completed);
                ACCESS_ONCE(rnp->completed) = rsp->completed;
                if (rnp == rdp->mynode)
                        (void)__note_gp_changes(rsp, rnp, rdp);
                rcu_preempt_boost_start_gp(rnp);
                trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
                                            rnp->level, rnp->grplo,
                                            rnp->grphi, rnp->qsmask);
                raw_spin_unlock_irq(&rnp->lock);
#ifdef CONFIG_PROVE_RCU_DELAY
                if ((prandom_u32() % (rcu_num_nodes + 1)) == 0 &&
                    system_state == SYSTEM_RUNNING)
                        udelay(200);
#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
                cond_resched();
        }

        mutex_unlock(&rsp->onoff_mutex);
        return 1;
}

/*
 * Do one round of quiescent-state forcing.
 */
static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
{
        int fqs_state = fqs_state_in;
        bool isidle = false;
        unsigned long maxj;
        struct rcu_node *rnp = rcu_get_root(rsp);

        rsp->n_force_qs++;
        if (fqs_state == RCU_SAVE_DYNTICK) {
                /* Collect dyntick-idle snapshots. */
                if (is_sysidle_rcu_state(rsp)) {
                        isidle = 1;
                        maxj = jiffies - ULONG_MAX / 4;
                }
                force_qs_rnp(rsp, dyntick_save_progress_counter,
                             &isidle, &maxj);
                rcu_sysidle_report_gp(rsp, isidle, maxj);
                fqs_state = RCU_FORCE_QS;
        } else {
                /* Handle dyntick-idle and offline CPUs. */
                isidle = 0;
                force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
        }
        /* Clear flag to prevent immediate re-entry. */
        if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
                raw_spin_lock_irq(&rnp->lock);
                smp_mb__after_unlock_lock();
                ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
                raw_spin_unlock_irq(&rnp->lock);
        }
        return fqs_state;
}

/*
 * Clean up after the old grace period.
 */
static void rcu_gp_cleanup(struct rcu_state *rsp)
{
        unsigned long gp_duration;
        bool needgp = false;
        int nocb = 0;
        struct rcu_data *rdp;
        struct rcu_node *rnp = rcu_get_root(rsp);

        raw_spin_lock_irq(&rnp->lock);
        smp_mb__after_unlock_lock();
        gp_duration = jiffies - rsp->gp_start;
        if (gp_duration > rsp->gp_max)
                rsp->gp_max = gp_duration;

        /*
         * We know the grace period is complete, but to everyone else
         * it appears to still be ongoing.  But it is also the case
         * that to everyone else it looks like there is nothing that
         * they can do to advance the grace period.  It is therefore
         * safe for us to drop the lock in order to mark the grace
         * period as completed in all of the rcu_node structures.
         */
        raw_spin_unlock_irq(&rnp->lock);

        /*
         * Propagate new ->completed value to rcu_node structures so
         * that other CPUs don't have to wait until the start of the next
         * grace period to process their callbacks.  This also avoids
         * some nasty RCU grace-period initialization races by forcing
         * the end of the current grace period to be completely recorded in
         * all of the rcu_node structures before the beginning of the next
         * grace period is recorded in any of the rcu_node structures.
         */
        rcu_for_each_node_breadth_first(rsp, rnp) {
                raw_spin_lock_irq(&rnp->lock);
                smp_mb__after_unlock_lock();
                ACCESS_ONCE(rnp->completed) = rsp->gpnum;
                rdp = this_cpu_ptr(rsp->rda);
                if (rnp == rdp->mynode)
                        needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
                /* smp_mb() provided by prior unlock-lock pair. */
                nocb += rcu_future_gp_cleanup(rsp, rnp);
                raw_spin_unlock_irq(&rnp->lock);
                cond_resched();
        }
        rnp = rcu_get_root(rsp);
        raw_spin_lock_irq(&rnp->lock);
        smp_mb__after_unlock_lock(); /* Order GP before ->completed update. */
        rcu_nocb_gp_set(rnp, nocb);

        /* Declare grace period done. */
        ACCESS_ONCE(rsp->completed) = rsp->gpnum;
        trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
        rsp->fqs_state = RCU_GP_IDLE;
        rdp = this_cpu_ptr(rsp->rda);
        /* Advance CBs to reduce false positives below. */
        needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
        if (needgp || cpu_needs_another_gp(rsp, rdp)) {
                ACCESS_ONCE(rsp->gp_flags) = RCU_GP_FLAG_INIT;
                trace_rcu_grace_period(rsp->name,
                                       ACCESS_ONCE(rsp->gpnum),
                                       TPS("newreq"));
        }
        raw_spin_unlock_irq(&rnp->lock);
}

/*
 * Body of kthread that handles grace periods.
 */
static int __noreturn rcu_gp_kthread(void *arg)
{
        int fqs_state;
        int gf;
        unsigned long j;
        int ret;
        struct rcu_state *rsp = arg;
        struct rcu_node *rnp = rcu_get_root(rsp);

        for (;;) {

                /* Handle grace-period start. */
                for (;;) {
                        trace_rcu_grace_period(rsp->name,
                                               ACCESS_ONCE(rsp->gpnum),
                                               TPS("reqwait"));
                        rsp->gp_state = RCU_GP_WAIT_GPS;
                        wait_event_interruptible(rsp->gp_wq,
                                                 ACCESS_ONCE(rsp->gp_flags) &
                                                 RCU_GP_FLAG_INIT);
                        /* Locking provides needed memory barrier. */
                        if (rcu_gp_init(rsp))
                                break;
                        cond_resched();
                        flush_signals(current);
                        trace_rcu_grace_period(rsp->name,
                                               ACCESS_ONCE(rsp->gpnum),
                                               TPS("reqwaitsig"));
                }

                /* Handle quiescent-state forcing. */
                fqs_state = RCU_SAVE_DYNTICK;
                j = jiffies_till_first_fqs;
                if (j > HZ) {
                        j = HZ;
                        jiffies_till_first_fqs = HZ;
                }
                ret = 0;
                for (;;) {
                        if (!ret)
                                rsp->jiffies_force_qs = jiffies + j;
                        trace_rcu_grace_period(rsp->name,
                                               ACCESS_ONCE(rsp->gpnum),
                                               TPS("fqswait"));
                        rsp->gp_state = RCU_GP_WAIT_FQS;
                        ret = wait_event_interruptible_timeout(rsp->gp_wq,
                                        ((gf = ACCESS_ONCE(rsp->gp_flags)) &
                                         RCU_GP_FLAG_FQS) ||
                                        (!ACCESS_ONCE(rnp->qsmask) &&
                                         !rcu_preempt_blocked_readers_cgp(rnp)),
                                        j);
                        /* Locking provides needed memory barriers. */
                        /* If grace period done, leave loop. */
                        if (!ACCESS_ONCE(rnp->qsmask) &&
                            !rcu_preempt_blocked_readers_cgp(rnp))
                                break;
                        /* If time for quiescent-state forcing, do it. */
                        if (ULONG_CMP_GE(jiffies, rsp->jiffies_force_qs) ||
                            (gf & RCU_GP_FLAG_FQS)) {
                                trace_rcu_grace_period(rsp->name,
                                                       ACCESS_ONCE(rsp->gpnum),
                                                       TPS("fqsstart"));
                                fqs_state = rcu_gp_fqs(rsp, fqs_state);
                                trace_rcu_grace_period(rsp->name,
                                                       ACCESS_ONCE(rsp->gpnum),
                                                       TPS("fqsend"));
                                cond_resched();
                        } else {
                                /* Deal with stray signal. */
                                cond_resched();
                                flush_signals(current);
                                trace_rcu_grace_period(rsp->name,
                                                       ACCESS_ONCE(rsp->gpnum),
                                                       TPS("fqswaitsig"));
                        }
                        j = jiffies_till_next_fqs;
                        if (j > HZ) {
                                j = HZ;
                                jiffies_till_next_fqs = HZ;
                        } else if (j < 1) {
                                j = 1;
                                jiffies_till_next_fqs = 1;
                        }
                }

                /* Handle grace-period end. */
                rcu_gp_cleanup(rsp);
        }
}

/*
 * Start a new RCU grace period if warranted, re-initializing the hierarchy
 * in preparation for detecting the next grace period.  The caller must hold
 * the root node's ->lock and hard irqs must be disabled.
 *
 * Note that it is legal for a dying CPU (which is marked as offline) to
 * invoke this function.  This can happen when the dying CPU reports its
 * quiescent state.
 *
 * Returns true if the grace-period kthread must be awakened.
 */
static bool
rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
                      struct rcu_data *rdp)
{
        if (!rsp->gp_kthread || !cpu_needs_another_gp(rsp, rdp)) {
                /*
                 * Either we have not yet spawned the grace-period
                 * task, this CPU does not need another grace period,
                 * or a grace period is already in progress.
                 * Either way, don't start a new grace period.
                 */
                return false;
        }
        ACCESS_ONCE(rsp->gp_flags) = RCU_GP_FLAG_INIT;
        trace_rcu_grace_period(rsp->name, ACCESS_ONCE(rsp->gpnum),
                               TPS("newreq"));

        /*
         * We can't do wakeups while holding the rnp->lock, as that
         * could cause possible deadlocks with the rq->lock. Defer
         * the wakeup to our caller.
         */
        return true;
}

/*
 * Similar to rcu_start_gp_advanced(), but also advance the calling CPU's
 * callbacks.  Note that rcu_start_gp_advanced() cannot do this because it
 * is invoked indirectly from rcu_advance_cbs(), which would result in
 * endless recursion -- or would do so if it wasn't for the self-deadlock
 * that is encountered beforehand.
 *
 * Returns true if the grace-period kthread needs to be awakened.
 */
static bool rcu_start_gp(struct rcu_state *rsp)
{
        struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
        struct rcu_node *rnp = rcu_get_root(rsp);
        bool ret = false;

        /*
         * If there is no grace period in progress right now, any
         * callbacks we have up to this point will be satisfied by the
         * next grace period.  Also, advancing the callbacks reduces the
         * probability of false positives from cpu_needs_another_gp()
         * resulting in pointless grace periods.  So, advance callbacks
         * then start the grace period!
         */
        ret = rcu_advance_cbs(rsp, rnp, rdp) || ret;
        ret = rcu_start_gp_advanced(rsp, rnp, rdp) || ret;
        return ret;
}

/*
 * Report a full set of quiescent states to the specified rcu_state
 * data structure.  This involves cleaning up after the prior grace
 * period and letting rcu_start_gp() start up the next grace period
 * if one is needed.  Note that the caller must hold rnp->lock, which
 * is released before return.
 */
static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
        __releases(rcu_get_root(rsp)->lock)
{
        WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
        raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
        wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
}

/*
 * Similar to rcu_report_qs_rdp(), for which it is a helper function.
 * Allows quiescent states for a group of CPUs to be reported at one go
 * to the specified rcu_node structure, though all the CPUs in the group
 * must be represented by the same rcu_node structure (which need not be
 * a leaf rcu_node structure, though it often will be).  That structure's
 * lock must be held upon entry, and it is released before return.
 */
static void
rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
                  struct rcu_node *rnp, unsigned long flags)
        __releases(rnp->lock)
{
        struct rcu_node *rnp_c;

        /* Walk up the rcu_node hierarchy. */
        for (;;) {
                if (!(rnp->qsmask & mask)) {

                        /* Our bit has already been cleared, so done. */
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                rnp->qsmask &= ~mask;
                trace_rcu_quiescent_state_report(rsp->name, rnp->gpnum,
                                                 mask, rnp->qsmask, rnp->level,
                                                 rnp->grplo, rnp->grphi,
                                                 !!rnp->gp_tasks);
                if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {

                        /* Other bits still set at this level, so done. */
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                mask = rnp->grpmask;
                if (rnp->parent == NULL) {

                        /* No more levels.  Exit loop holding root lock. */

                        break;
                }
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                rnp_c = rnp;
                rnp = rnp->parent;
                raw_spin_lock_irqsave(&rnp->lock, flags);
                smp_mb__after_unlock_lock();
                WARN_ON_ONCE(rnp_c->qsmask);
        }

        /*
         * Get here if we are the last CPU to pass through a quiescent
         * state for this grace period.  Invoke rcu_report_qs_rsp()
         * to clean up and start the next grace period if one is needed.
         */
        rcu_report_qs_rsp(rsp, flags); /* releases rnp->lock. */
}

/*
 * Record a quiescent state for the specified CPU to that CPU's rcu_data
 * structure.  This must be either called from the specified CPU, or
 * called when the specified CPU is known to be offline (and when it is
 * also known that no other CPU is concurrently trying to help the offline
 * CPU).  The lastcomp argument is used to make sure we are still in the
 * grace period of interest.  We don't want to end the current grace period
 * based on quiescent states detected in an earlier grace period!
 */
static void
rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        unsigned long mask;
        bool needwake;
        struct rcu_node *rnp;

        rnp = rdp->mynode;
        raw_spin_lock_irqsave(&rnp->lock, flags);
        smp_mb__after_unlock_lock();
        if (rdp->passed_quiesce == 0 || rdp->gpnum != rnp->gpnum ||
            rnp->completed == rnp->gpnum) {

                /*
                 * The grace period in which this quiescent state was
                 * recorded has ended, so don't report it upwards.
                 * We will instead need a new quiescent state that lies
                 * within the current grace period.
                 */
                rdp->passed_quiesce = 0;        /* need qs for new gp. */
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                return;
        }
        mask = rdp->grpmask;
        if ((rnp->qsmask & mask) == 0) {
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        } else {
                rdp->qs_pending = 0;

                /*
                 * This GP can't end until cpu checks in, so all of our
                 * callbacks can be processed during the next GP.
                 */
                needwake = rcu_accelerate_cbs(rsp, rnp, rdp);

                rcu_report_qs_rnp(mask, rsp, rnp, flags); /* rlses rnp->lock */
                if (needwake)
                        rcu_gp_kthread_wake(rsp);
        }
}

/*
 * Check to see if there is a new grace period of which this CPU
 * is not yet aware, and if so, set up local rcu_data state for it.
 * Otherwise, see if this CPU has just passed through its first
 * quiescent state for this grace period, and record that fact if so.
 */
static void
rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
{
        /* Check for grace-period ends and beginnings. */
        note_gp_changes(rsp, rdp);

        /*
         * Does this CPU still need to do its part for current grace period?
         * If no, return and let the other CPUs do their part as well.
         */
        if (!rdp->qs_pending)
                return;

        /*
         * Was there a quiescent state since the beginning of the grace
         * period? If no, then exit and wait for the next call.
         */
        if (!rdp->passed_quiesce)
                return;

        /*
         * Tell RCU we are done (but rcu_report_qs_rdp() will be the
         * judge of that).
         */
        rcu_report_qs_rdp(rdp->cpu, rsp, rdp);
}

#ifdef CONFIG_HOTPLUG_CPU

/*
 * Send the specified CPU's RCU callbacks to the orphanage.  The
 * specified CPU must be offline, and the caller must hold the
 * ->orphan_lock.
 */
static void
rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
                          struct rcu_node *rnp, struct rcu_data *rdp)
{
        /* No-CBs CPUs do not have orphanable callbacks. */
        if (rcu_is_nocb_cpu(rdp->cpu))
                return;

        /*
         * Orphan the callbacks.  First adjust the counts.  This is safe
         * because _rcu_barrier() excludes CPU-hotplug operations, so it
         * cannot be running now.  Thus no memory barrier is required.
         */
        if (rdp->nxtlist != NULL) {
                rsp->qlen_lazy += rdp->qlen_lazy;
                rsp->qlen += rdp->qlen;
                rdp->n_cbs_orphaned += rdp->qlen;
                rdp->qlen_lazy = 0;
                ACCESS_ONCE(rdp->qlen) = 0;
        }

        /*
         * Next, move those callbacks still needing a grace period to
         * the orphanage, where some other CPU will pick them up.
         * Some of the callbacks might have gone partway through a grace
         * period, but that is too bad.  They get to start over because we
         * cannot assume that grace periods are synchronized across CPUs.
         * We don't bother updating the ->nxttail[] array yet, instead
         * we just reset the whole thing later on.
         */
        if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
                *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
                rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
                *rdp->nxttail[RCU_DONE_TAIL] = NULL;
        }

        /*
         * Then move the ready-to-invoke callbacks to the orphanage,
         * where some other CPU will pick them up.  These will not be
         * required to pass though another grace period: They are done.
         */
        if (rdp->nxtlist != NULL) {
                *rsp->orphan_donetail = rdp->nxtlist;
                rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
        }

        /* Finally, initialize the rcu_data structure's list to empty.  */
        init_callback_list(rdp);
}

/*
 * Adopt the RCU callbacks from the specified rcu_state structure's
 * orphanage.  The caller must hold the ->orphan_lock.
 */
static void rcu_adopt_orphan_cbs(struct rcu_state *rsp, unsigned long flags)
{
        int i;
        struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);

        /* No-CBs CPUs are handled specially. */
        if (rcu_nocb_adopt_orphan_cbs(rsp, rdp, flags))
                return;

        /* Do the accounting first. */
        rdp->qlen_lazy += rsp->qlen_lazy;
        rdp->qlen += rsp->qlen;
        rdp->n_cbs_adopted += rsp->qlen;
        if (rsp->qlen_lazy != rsp->qlen)
                rcu_idle_count_callbacks_posted();
        rsp->qlen_lazy = 0;
        rsp->qlen = 0;

        /*
         * We do not need a memory barrier here because the only way we
         * can get here if there is an rcu_barrier() in flight is if
         * we are the task doing the rcu_barrier().
         */

        /* First adopt the ready-to-invoke callbacks. */
        if (rsp->orphan_donelist != NULL) {
                *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
                *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
                for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
                        if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
                                rdp->nxttail[i] = rsp->orphan_donetail;
                rsp->orphan_donelist = NULL;
                rsp->orphan_donetail = &rsp->orphan_donelist;
        }

        /* And then adopt the callbacks that still need a grace period. */
        if (rsp->orphan_nxtlist != NULL) {
                *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
                rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
                rsp->orphan_nxtlist = NULL;
                rsp->orphan_nxttail = &rsp->orphan_nxtlist;
        }
}

/*
 * Trace the fact that this CPU is going offline.
 */
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
        RCU_TRACE(unsigned long mask);
        RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
        RCU_TRACE(struct rcu_node *rnp = rdp->mynode);

        RCU_TRACE(mask = rdp->grpmask);
        trace_rcu_grace_period(rsp->name,
                               rnp->gpnum + 1 - !!(rnp->qsmask & mask),
                               TPS("cpuofl"));
}

/*
 * The CPU has been completely removed, and some other CPU is reporting
 * this fact from process context.  Do the remainder of the cleanup,
 * including orphaning the outgoing CPU's RCU callbacks, and also
 * adopting them.  There can only be one CPU hotplug operation at a time,
 * so no other CPU can be attempting to update rcu_cpu_kthread_task.
 */
static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
{
        unsigned long flags;
        unsigned long mask;
        int need_report = 0;
        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
        struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */

        /* Adjust any no-longer-needed kthreads. */
        rcu_boost_kthread_setaffinity(rnp, -1);

        /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */

        /* Exclude any attempts to start a new grace period. */
        mutex_lock(&rsp->onoff_mutex);
        raw_spin_lock_irqsave(&rsp->orphan_lock, flags);

        /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
        rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
        rcu_adopt_orphan_cbs(rsp, flags);

        /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
        mask = rdp->grpmask;    /* rnp->grplo is constant. */
        do {
                raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
                smp_mb__after_unlock_lock();
                rnp->qsmaskinit &= ~mask;
                if (rnp->qsmaskinit != 0) {
                        if (rnp != rdp->mynode)
                                raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                        break;
                }
                if (rnp == rdp->mynode)
                        need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
                else
                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
                mask = rnp->grpmask;
                rnp = rnp->parent;
        } while (rnp != NULL);

        /*
         * We still hold the leaf rcu_node structure lock here, and
         * irqs are still disabled.  The reason for this subterfuge is
         * because invoking rcu_report_unblock_qs_rnp() with ->orphan_lock
         * held leads to deadlock.
         */
        raw_spin_unlock(&rsp->orphan_lock); /* irqs remain disabled. */
        rnp = rdp->mynode;
        if (need_report & RCU_OFL_TASKS_NORM_GP)
                rcu_report_unblock_qs_rnp(rnp, flags);
        else
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        if (need_report & RCU_OFL_TASKS_EXP_GP)
                rcu_report_exp_rnp(rsp, rnp, true);
        WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL,
                  "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n",
                  cpu, rdp->qlen, rdp->nxtlist);
        init_callback_list(rdp);
        /* Disallow further callbacks on this CPU. */
        rdp->nxttail[RCU_NEXT_TAIL] = NULL;
        mutex_unlock(&rsp->onoff_mutex);
}

#else /* #ifdef CONFIG_HOTPLUG_CPU */

static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
}

static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
{
}

#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */

/*
 * Invoke any RCU callbacks that have made it to the end of their grace
 * period.  Thottle as specified by rdp->blimit.
 */
static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
{
        unsigned long flags;
        struct rcu_head *next, *list, **tail;
        long bl, count, count_lazy;
        int i;

        /* If no callbacks are ready, just return. */
        if (!cpu_has_callbacks_ready_to_invoke(rdp)) {
                trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, rdp->qlen, 0);
                trace_rcu_batch_end(rsp->name, 0, !!ACCESS_ONCE(rdp->nxtlist),
                                    need_resched(), is_idle_task(current),
                                    rcu_is_callbacks_kthread());
                return;
        }

        /*
         * Extract the list of ready callbacks, disabling to prevent
         * races with call_rcu() from interrupt handlers.
         */
        local_irq_save(flags);
        WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
        bl = rdp->blimit;
        trace_rcu_batch_start(rsp->name, rdp->qlen_lazy, rdp->qlen, bl);
        list = rdp->nxtlist;
        rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
        *rdp->nxttail[RCU_DONE_TAIL] = NULL;
        tail = rdp->nxttail[RCU_DONE_TAIL];
        for (i = RCU_NEXT_SIZE - 1; i >= 0; i--)
                if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
                        rdp->nxttail[i] = &rdp->nxtlist;
        local_irq_restore(flags);

        /* Invoke callbacks. */
        count = count_lazy = 0;
        while (list) {
                next = list->next;
                prefetch(next);
                debug_rcu_head_unqueue(list);
                if (__rcu_reclaim(rsp->name, list))
                        count_lazy++;
                list = next;
                /* Stop only if limit reached and CPU has something to do. */
                if (++count >= bl &&
                    (need_resched() ||
                     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
                        break;
        }

        local_irq_save(flags);
        trace_rcu_batch_end(rsp->name, count, !!list, need_resched(),
                            is_idle_task(current),
                            rcu_is_callbacks_kthread());

        /* Update count, and requeue any remaining callbacks. */
        if (list != NULL) {
                *tail = rdp->nxtlist;
                rdp->nxtlist = list;
                for (i = 0; i < RCU_NEXT_SIZE; i++)
                        if (&rdp->nxtlist == rdp->nxttail[i])
                                rdp->nxttail[i] = tail;
                        else
                                break;
        }
        smp_mb(); /* List handling before counting for rcu_barrier(). */
        rdp->qlen_lazy -= count_lazy;
        ACCESS_ONCE(rdp->qlen) -= count;
        rdp->n_cbs_invoked += count;

        /* Reinstate batch limit if we have worked down the excess. */
        if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
                rdp->blimit = blimit;

        /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
        if (rdp->qlen == 0 && rdp->qlen_last_fqs_check != 0) {
                rdp->qlen_last_fqs_check = 0;
                rdp->n_force_qs_snap = rsp->n_force_qs;
        } else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
                rdp->qlen_last_fqs_check = rdp->qlen;
        WARN_ON_ONCE((rdp->nxtlist == NULL) != (rdp->qlen == 0));

        local_irq_restore(flags);

        /* Re-invoke RCU core processing if there are callbacks remaining. */
        if (cpu_has_callbacks_ready_to_invoke(rdp))
                invoke_rcu_core();
}

/*
 * Check to see if this CPU is in a non-context-switch quiescent state
 * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
 * Also schedule RCU core processing.
 *
 * This function must be called from hardirq context.  It is normally
 * invoked from the scheduling-clock interrupt.  If rcu_pending returns
 * false, there is no point in invoking rcu_check_callbacks().
 */
void rcu_check_callbacks(int cpu, int user)
{
        trace_rcu_utilization(TPS("Start scheduler-tick"));
        increment_cpu_stall_ticks();
        if (user || rcu_is_cpu_rrupt_from_idle()) {

                /*
                 * Get here if this CPU took its interrupt from user
                 * mode or from the idle loop, and if this is not a
                 * nested interrupt.  In this case, the CPU is in
                 * a quiescent state, so note it.
                 *
                 * No memory barrier is required here because both
                 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
                 * variables that other CPUs neither access nor modify,
                 * at least not while the corresponding CPU is online.
                 */

                rcu_sched_qs(cpu);
                rcu_bh_qs(cpu);

        } else if (!in_softirq()) {

                /*
                 * Get here if this CPU did not take its interrupt from
                 * softirq, in other words, if it is not interrupting
                 * a rcu_bh read-side critical section.  This is an _bh
                 * critical section, so note it.
                 */

                rcu_bh_qs(cpu);
        }
        rcu_preempt_check_callbacks(cpu);
        if (rcu_pending(cpu))
                invoke_rcu_core();
        trace_rcu_utilization(TPS("End scheduler-tick"));
}

/*
 * Scan the leaf rcu_node structures, processing dyntick state for any that
 * have not yet encountered a quiescent state, using the function specified.
 * Also initiate boosting for any threads blocked on the root rcu_node.
 *
 * The caller must have suppressed start of new grace periods.
 */
static void force_qs_rnp(struct rcu_state *rsp,
                         int (*f)(struct rcu_data *rsp, bool *isidle,
                                  unsigned long *maxj),
                         bool *isidle, unsigned long *maxj)
{
        unsigned long bit;
        int cpu;
        unsigned long flags;
        unsigned long mask;
        struct rcu_node *rnp;

        rcu_for_each_leaf_node(rsp, rnp) {
                cond_resched();
                mask = 0;
                raw_spin_lock_irqsave(&rnp->lock, flags);
                smp_mb__after_unlock_lock();
                if (!rcu_gp_in_progress(rsp)) {
                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
                }
                if (rnp->qsmask == 0) {
                        rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
                        continue;
                }
                cpu = rnp->grplo;
                bit = 1;
                for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
                        if ((rnp->qsmask & bit) != 0) {
                                if ((rnp->qsmaskinit & bit) != 0)
                                        *isidle = 0;
                                if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
                                        mask |= bit;
                        }
                }
                if (mask != 0) {

                        /* rcu_report_qs_rnp() releases rnp->lock. */
                        rcu_report_qs_rnp(mask, rsp, rnp, flags);
                        continue;
                }
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }
        rnp = rcu_get_root(rsp);
        if (rnp->qsmask == 0) {
                raw_spin_lock_irqsave(&rnp->lock, flags);
                smp_mb__after_unlock_lock();
                rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
        }
}

/*
 * Force quiescent states on reluctant CPUs, and also detect which
 * CPUs are in dyntick-idle mode.
 */
static void force_quiescent_state(struct rcu_state *rsp)
{
        unsigned long flags;
        bool ret;
        struct rcu_node *rnp;
        struct rcu_node *rnp_old = NULL;

        /* Funnel through hierarchy to reduce memory contention. */
        rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
        for (; rnp != NULL; rnp = rnp->parent) {
                ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
                      !raw_spin_trylock(&rnp->fqslock);
                if (rnp_old != NULL)
                        raw_spin_unlock(&rnp_old->fqslock);
                if (ret) {
                        ACCESS_ONCE(rsp->n_force_qs_lh)++;
                        return;
                }
                rnp_old = rnp;
        }
        /* rnp_old == rcu_get_root(rsp), rnp == NULL. */

        /* Reached the root of the rcu_node tree, acquire lock. */
        raw_spin_lock_irqsave(&rnp_old->lock, flags);
        smp_mb__after_unlock_lock();
        raw_spin_unlock(&rnp_old->fqslock);
        if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
                ACCESS_ONCE(rsp->n_force_qs_lh)++;
                raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
                return;  /* Someone beat us to it. */
        }
        ACCESS_ONCE(rsp->gp_flags) |= RCU_GP_FLAG_FQS;
        raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
        wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
}

/*
 * This does the RCU core processing work for the specified rcu_state
 * and rcu_data structures.  This may be called only from the CPU to
 * whom the rdp belongs.
 */
static void
__rcu_process_callbacks(struct rcu_state *rsp)
{
        unsigned long flags;
        bool needwake;
        struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);

        WARN_ON_ONCE(rdp->beenonline == 0);

        /* Update RCU state based on any recent quiescent states. */
        rcu_check_quiescent_state(rsp, rdp);

        /* Does this CPU require a not-yet-started grace period? */
        local_irq_save(flags);
        if (cpu_needs_another_gp(rsp, rdp)) {
                raw_spin_lock(&rcu_get_root(rsp)->lock); /* irqs disabled. */
                needwake = rcu_start_gp(rsp);
                raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
                if (needwake)
                        rcu_gp_kthread_wake(rsp);
        } else {
                local_irq_restore(flags);
        }

        /* If there are callbacks ready, invoke them. */
        if (cpu_has_callbacks_ready_to_invoke(rdp))
                invoke_rcu_callbacks(rsp, rdp);

        /* Do any needed deferred wakeups of rcuo kthreads. */
        do_nocb_deferred_wakeup(rdp);
}

/*
 * Do RCU core processing for the current CPU.
 */
static void rcu_process_callbacks(struct softirq_action *unused)
{
        struct rcu_state *rsp;

        if (cpu_is_offline(smp_processor_id()))
                return;
        trace_rcu_utilization(TPS("Start RCU core"));
        for_each_rcu_flavor(rsp)
                __rcu_process_callbacks(rsp);
        trace_rcu_utilization(TPS("End RCU core"));
}

/*
 * Schedule RCU callback invocation.  If the specified type of RCU
 * does not support RCU priority boosting, just do a direct call,
 * otherwise wake up the per-CPU kernel kthread.  Note that because we
 * are running on the current CPU with interrupts disabled, the
 * rcu_cpu_kthread_task cannot disappear out from under us.
 */
static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
{
        if (unlikely(!ACCESS_ONCE(rcu_scheduler_fully_active)))
                return;
        if (likely(!rsp->boost)) {
                rcu_do_batch(rsp, rdp);
                return;
        }
        invoke_rcu_callbacks_kthread();
}

static void invoke_rcu_core(void)
{
        if (cpu_online(smp_processor_id()))
                raise_softirq(RCU_SOFTIRQ);
}

/*
 * Handle any core-RCU processing required by a call_rcu() invocation.
 */
static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
                            struct rcu_head *head, unsigned long flags)
{
        bool needwake;

        /*
         * If called from an extended quiescent state, invoke the RCU
         * core in order to force a re-evaluation of RCU's idleness.
         */
        if (!rcu_is_watching() && cpu_online(smp_processor_id()))
                invoke_rcu_core();

        /* If interrupts were disabled or CPU offline, don't invoke RCU core. */
        if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id()))
                return;

        /*
         * Force the grace period if too many callbacks or too long waiting.
         * Enforce hysteresis, and don't invoke force_quiescent_state()
         * if some other CPU has recently done so.  Also, don't bother
         * invoking force_quiescent_state() if the newly enqueued callback
         * is the only one waiting for a grace period to complete.
         */
        if (unlikely(rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {

                /* Are we ignoring a completed grace period? */
                note_gp_changes(rsp, rdp);

                /* Start a new grace period if one not already started. */
                if (!rcu_gp_in_progress(rsp)) {
                        struct rcu_node *rnp_root = rcu_get_root(rsp);

                        raw_spin_lock(&rnp_root->lock);
                        smp_mb__after_unlock_lock();
                        needwake = rcu_start_gp(rsp);
                        raw_spin_unlock(&rnp_root->lock);
                        if (needwake)
                                rcu_gp_kthread_wake(rsp);
                } else {
                        /* Give the grace period a kick. */
                        rdp->blimit = LONG_MAX;
                        if (rsp->n_force_qs == rdp->n_force_qs_snap &&
                            *rdp->nxttail[RCU_DONE_TAIL] != head)
                                force_quiescent_state(rsp);
                        rdp->n_force_qs_snap = rsp->n_force_qs;
                        rdp->qlen_last_fqs_check = rdp->qlen;
                }
        }
}

/*
 * RCU callback function to leak a callback.
 */
static void rcu_leak_callback(struct rcu_head *rhp)
{
}

/*
 * Helper function for call_rcu() and friends.  The cpu argument will
 * normally be -1, indicating "currently running CPU".  It may specify
 * a CPU only if that CPU is a no-CBs CPU.  Currently, only _rcu_barrier()
 * is expected to specify a CPU.
 */
static void
__call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
           struct rcu_state *rsp, int cpu, bool lazy)
{
        unsigned long flags;
        struct rcu_data *rdp;

        WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
        if (debug_rcu_head_queue(head)) {
                /* Probable double call_rcu(), so leak the callback. */
                ACCESS_ONCE(head->func) = rcu_leak_callback;
                WARN_ONCE(1, "__call_rcu(): Leaked duplicate callback\n");
                return;
        }
        head->func = func;
        head->next = NULL;

        /*
         * Opportunistically note grace-period endings and beginnings.
         * Note that we might see a beginning right after we see an
         * end, but never vice versa, since this CPU has to pass through
         * a quiescent state betweentimes.
         */
        local_irq_save(flags);
        rdp = this_cpu_ptr(rsp->rda);

        /* Add the callback to our list. */
        if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL) || cpu != -1) {
                int offline;

                if (cpu != -1)
                        rdp = per_cpu_ptr(rsp->rda, cpu);
                offline = !__call_rcu_nocb(rdp, head, lazy, flags);
                WARN_ON_ONCE(offline);
                /* _call_rcu() is illegal on offline CPU; leak the callback. */
                local_irq_restore(flags);
                return;
        }
        ACCESS_ONCE(rdp->qlen)++;
        if (lazy)
                rdp->qlen_lazy++;
        else
                rcu_idle_count_callbacks_posted();
        smp_mb();  /* Count before adding callback for rcu_barrier(). */
        *rdp->nxttail[RCU_NEXT_TAIL] = head;
        rdp->nxttail[RCU_NEXT_TAIL] = &head->next;

        if (__is_kfree_rcu_offset((unsigned long)func))
                trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
                                         rdp->qlen_lazy, rdp->qlen);
        else
                trace_rcu_callback(rsp->name, head, rdp->qlen_lazy, rdp->qlen);

        /* Go handle any RCU core processing required. */
        __call_rcu_core(rsp, rdp, head, flags);
        local_irq_restore(flags);
}

/*
 * Queue an RCU-sched callback for invocation after a grace period.
 */
void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_sched_state, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu_sched);

/*
 * Queue an RCU callback for invocation after a quicker grace period.
 */
void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, &rcu_bh_state, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

/*
 * Queue an RCU callback for lazy invocation after a grace period.
 * This will likely be later named something like "call_rcu_lazy()",
 * but this change will require some way of tagging the lazy RCU
 * callbacks in the list of pending callbacks. Until then, this
 * function may only be called from __kfree_rcu().
 */
void kfree_call_rcu(struct rcu_head *head,
                    void (*func)(struct rcu_head *rcu))
{
        __call_rcu(head, func, rcu_state_p, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

/*
 * Because a context switch is a grace period for RCU-sched and RCU-bh,
 * any blocking grace-period wait automatically implies a grace period
 * if there is only one CPU online at any point time during execution
 * of either synchronize_sched() or synchronize_rcu_bh().  It is OK to
 * occasionally incorrectly indicate that there are multiple CPUs online
 * when there was in fact only one the whole time, as this just adds
 * some overhead: RCU still operates correctly.
 */
static inline int rcu_blocking_is_gp(void)
{
        int ret;

        might_sleep();  /* Check for RCU read-side critical section. */
        preempt_disable();
        ret = num_online_cpus() <= 1;
        preempt_enable();
        return ret;
}

/**
 * synchronize_sched - wait until an rcu-sched grace period has elapsed.
 *
 * Control will return to the caller some time after a full rcu-sched
 * grace period has elapsed, in other words after all currently executing
 * rcu-sched read-side critical sections have completed.   These read-side
 * critical sections are delimited by rcu_read_lock_sched() and
 * rcu_read_unlock_sched(), and may be nested.  Note that preempt_disable(),
 * local_irq_disable(), and so on may be used in place of
 * rcu_read_lock_sched().
 *
 * This means that all preempt_disable code sequences, including NMI and
 * non-threaded hardware-interrupt handlers, in progress on entry will
 * have completed before this primitive returns.  However, this does not
 * guarantee that softirq handlers will have completed, since in some
 * kernels, these handlers can run in process context, and can block.
 *
 * Note that this guarantee implies further memory-ordering guarantees.
 * On systems with more than one CPU, when synchronize_sched() returns,
 * each CPU is guaranteed to have executed a full memory barrier since the
 * end of its last RCU-sched read-side critical section whose beginning
 * preceded the call to synchronize_sched().  In addition, each CPU having
 * an RCU read-side critical section that extends beyond the return from
 * synchronize_sched() is guaranteed to have executed a full memory barrier
 * after the beginning of synchronize_sched() and before the beginning of
 * that RCU read-side critical section.  Note that these guarantees include
 * CPUs that are offline, idle, or executing in user mode, as well as CPUs
 * that are executing in the kernel.
 *
 * Furthermore, if CPU A invoked synchronize_sched(), which returned
 * to its caller on CPU B, then both CPU A and CPU B are guaranteed
 * to have executed a full memory barrier during the execution of
 * synchronize_sched() -- even if CPU A and CPU B are the same CPU (but
 * again only if the system has more than one CPU).
 *
 * This primitive provides the guarantees made by the (now removed)
 * synchronize_kernel() API.  In contrast, synchronize_rcu() only
 * guarantees that rcu_read_lock() sections will have completed.
 * In "classic RCU", these two guarantees happen to be one and
 * the same, but can differ in realtime RCU implementations.
 */
void synchronize_sched(void)
{
        rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                           !lock_is_held(&rcu_lock_map) &&
                           !lock_is_held(&rcu_sched_lock_map),
                           "Illegal synchronize_sched() in RCU-sched read-side critical section");
        if (rcu_blocking_is_gp())
                return;
        if (rcu_expedited)
                synchronize_sched_expedited();
        else
                wait_rcu_gp(call_rcu_sched);
}
EXPORT_SYMBOL_GPL(synchronize_sched);

/**
 * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
 *
 * Control will return to the caller some time after a full rcu_bh grace
 * period has elapsed, in other words after all currently executing rcu_bh
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
 * and may be nested.
 *
 * See the description of synchronize_sched() for more detailed information
 * on memory ordering guarantees.
 */
void synchronize_rcu_bh(void)
{
        rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
                           !lock_is_held(&rcu_lock_map) &&
                           !lock_is_held(&rcu_sched_lock_map),
                           "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
        if (rcu_blocking_is_gp())
                return;
        if (rcu_expedited)
                synchronize_rcu_bh_expedited();
        else
                wait_rcu_gp(call_rcu_bh);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_bh);

/**
 * get_state_synchronize_rcu - Snapshot current RCU state
 *
 * Returns a cookie that is used by a later call to cond_synchronize_rcu()
 * to determine whether or not a full grace period has elapsed in the
 * meantime.
 */
unsigned long get_state_synchronize_rcu(void)
{
        /*
         * Any prior manipulation of RCU-protected data must happen
         * before the load from ->gpnum.
         */
        smp_mb();  /* ^^^ */

        /*
         * Make sure this load happens before the purportedly
         * time-consuming work between get_state_synchronize_rcu()
         * and cond_synchronize_rcu().
         */
        return smp_load_acquire(&rcu_state_p->gpnum);
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);

/**
 * cond_synchronize_rcu - Conditionally wait for an RCU grace period
 *
 * @oldstate: return value from earlier call to get_state_synchronize_rcu()
 *
 * If a full RCU grace period has elapsed since the earlier call to
 * get_state_synchronize_rcu(), just return.  Otherwise, invoke
 * synchronize_rcu() to wait for a full grace period.
 *
 * Yes, this function does not take counter wrap into account.  But
 * counter wrap is harmless.  If the counter wraps, we have waited for
 * more than 2 billion grace periods (and way more on a 64-bit system!),
 * so waiting for one additional grace period should be just fine.
 */
void cond_synchronize_rcu(unsigned long oldstate)
{
        unsigned long newstate;

        /*
         * Ensure that this load happens before any RCU-destructive
         * actions the caller might carry out after we return.
         */
        newstate = smp_load_acquire(&rcu_state_p->completed);
        if (ULONG_CMP_GE(oldstate, newstate))
                synchronize_rcu();
}
EXPORT_SYMBOL_GPL(cond_synchronize_rcu);

static int synchronize_sched_expedited_cpu_stop(void *data)
{
        /*
         * There must be a full memory barrier on each affected CPU
         * between the time that try_stop_cpus() is called and the
         * time that it returns.
         *
         * In the current initial implementation of cpu_stop, the
         * above condition is already met when the control reaches
         * this point and the following smp_mb() is not strictly
         * necessary.  Do smp_mb() anyway for documentation and
         * robustness against future implementation changes.
         */
        smp_mb(); /* See above comment block. */
        return 0;
}

/**
 * synchronize_sched_expedited - Brute-force RCU-sched grace period
 *
 * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
 * approach to force the grace period to end quickly.  This consumes
 * significant time on all CPUs and is unfriendly to real-time workloads,
 * so is thus not recommended for any sort of common-case code.  In fact,
 * if you are using synchronize_sched_expedited() in a loop, please
 * restructure your code to batch your updates, and then use a single
 * synchronize_sched() instead.
 *
 * Note that it is illegal to call this function while holding any lock
 * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
 * to call this function from a CPU-hotplug notifier.  Failing to observe
 * these restriction will result in deadlock.
 *
 * This implementation can be thought of as an application of ticket
 * locking to RCU, with sync_sched_expedited_started and
 * sync_sched_expedited_done taking on the roles of the halves
 * of the ticket-lock word.  Each task atomically increments
 * sync_sched_expedited_started upon entry, snapshotting the old value,
 * then attempts to stop all the CPUs.  If this succeeds, then each
 * CPU will have executed a context switch, resulting in an RCU-sched
 * grace period.  We are then done, so we use atomic_cmpxchg() to
 * update sync_sched_expedited_done to match our snapshot -- but
 * only if someone else has not already advanced past our snapshot.
 *
 * On the other hand, if try_stop_cpus() fails, we check the value
 * of sync_sched_expedited_done.  If it has advanced past our
 * initial snapshot, then someone else must have forced a grace period
 * some time after we took our snapshot.  In this case, our work is
 * done for us, and we can simply return.  Otherwise, we try again,
 * but keep our initial snapshot for purposes of checking for someone
 * doing our work for us.
 *
 * If we fail too many times in a row, we fall back to synchronize_sched().
 */
void synchronize_sched_expedited(void)
{
        long firstsnap, s, snap;
        int trycount = 0;
        struct rcu_state *rsp = &rcu_sched_state;

        /*
         * If we are in danger of counter wrap, just do synchronize_sched().
         * By allowing sync_sched_expedited_started to advance no more than
         * ULONG_MAX/8 ahead of sync_sched_expedited_done, we are ensuring
         * that more than 3.5 billion CPUs would be required to force a
         * counter wrap on a 32-bit system.  Quite a few more CPUs would of
         * course be required on a 64-bit system.
         */
        if (ULONG_CMP_GE((ulong)atomic_long_read(&rsp->expedited_start),
                         (ulong)atomic_long_read(&rsp->expedited_done) +
                         ULONG_MAX / 8)) {
                synchronize_sched();
                atomic_long_inc(&rsp->expedited_wrap);
                return;
        }

        /*
         * Take a ticket.  Note that atomic_inc_return() implies a
         * full memory barrier.
         */
        snap = atomic_long_inc_return(&rsp->expedited_start);
        firstsnap = snap;
        get_online_cpus();
        WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));

        /*
         * Each pass through the following loop attempts to force a
         * context switch on each CPU.
         */
        while (try_stop_cpus(cpu_online_mask,
                             synchronize_sched_expedited_cpu_stop,
                             NULL) == -EAGAIN) {
                put_online_cpus();
                atomic_long_inc(&rsp->expedited_tryfail);

                /* Check to see if someone else did our work for us. */
                s = atomic_long_read(&rsp->expedited_done);
                if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
                        /* ensure test happens before caller kfree */
                        smp_mb__before_atomic(); /* ^^^ */
                        atomic_long_inc(&rsp->expedited_workdone1);
                        return;
                }

                /* No joy, try again later.  Or just synchronize_sched(). */
                if (trycount++ < 10) {
                        udelay(trycount * num_online_cpus());
                } else {
                        wait_rcu_gp(call_rcu_sched);
                        atomic_long_inc(&rsp->expedited_normal);
                        return;
                }

                /* Recheck to see if someone else did our work for us. */
                s = atomic_long_read(&rsp->expedited_done);
                if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
                        /* ensure test happens before caller kfree */
                        smp_mb__before_atomic(); /* ^^^ */
                        atomic_long_inc(&rsp->expedited_workdone2);
                        return;
                }

                /*
                 * Refetching sync_sched_expedited_started allows later
                 * callers to piggyback on our grace period.  We retry
                 * after they started, so our grace period works for them,
                 * and they started after our first try, so their grace
                 * period works for us.
                 */
                get_online_cpus();
                snap = atomic_long_read(&rsp->expedited_start);
                smp_mb(); /* ensure read is before try_stop_cpus(). */
        }
        atomic_long_inc(&rsp->expedited_stoppedcpus);

        /*
         * Everyone up to our most recent fetch is covered by our grace
         * period.  Update the counter, but only if our work is still
         * relevant -- which it won't be if someone who started later
         * than we did already did their update.
         */
        do {
                atomic_long_inc(&rsp->expedited_done_tries);
                s = atomic_long_read(&rsp->expedited_done);
                if (ULONG_CMP_GE((ulong)s, (ulong)snap)) {
                        /* ensure test happens before caller kfree */
                        smp_mb__before_atomic(); /* ^^^ */
                        atomic_long_inc(&rsp->expedited_done_lost);
                        break;
                }
        } while (atomic_long_cmpxchg(&rsp->expedited_done, s, snap) != s);
        atomic_long_inc(&rsp->expedited_done_exit);

        put_online_cpus();
}
EXPORT_SYMBOL_GPL(synchronize_sched_expedited);

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, for the specified type of RCU, returning 1 if so.
 * The checks are in order of increasing expense: checks that can be
 * carried out against CPU-local state are performed first.  However,
 * we must check for CPU stalls first, else we might not get a chance.
 */
static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
{
        struct rcu_node *rnp = rdp->mynode;

        rdp->n_rcu_pending++;

        /* Check for CPU stalls, if enabled. */
        check_cpu_stall(rsp, rdp);

        /* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
        if (rcu_nohz_full_cpu(rsp))
                return 0;

        /* Is the RCU core waiting for a quiescent state from this CPU? */
        if (rcu_scheduler_fully_active &&
            rdp->qs_pending && !rdp->passed_quiesce) {
                rdp->n_rp_qs_pending++;
        } else if (rdp->qs_pending && rdp->passed_quiesce) {
                rdp->n_rp_report_qs++;
                return 1;
        }

        /* Does this CPU have callbacks ready to invoke? */
        if (cpu_has_callbacks_ready_to_invoke(rdp)) {
                rdp->n_rp_cb_ready++;
                return 1;
        }

        /* Has RCU gone idle with this CPU needing another grace period? */
        if (cpu_needs_another_gp(rsp, rdp)) {
                rdp->n_rp_cpu_needs_gp++;
                return 1;
        }

        /* Has another RCU grace period completed?  */
        if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */
                rdp->n_rp_gp_completed++;
                return 1;
        }

        /* Has a new RCU grace period started? */
        if (ACCESS_ONCE(rnp->gpnum) != rdp->gpnum) { /* outside lock */
                rdp->n_rp_gp_started++;
                return 1;
        }

        /* Does this CPU need a deferred NOCB wakeup? */
        if (rcu_nocb_need_deferred_wakeup(rdp)) {
                rdp->n_rp_nocb_defer_wakeup++;
                return 1;
        }

        /* nothing to do */
        rdp->n_rp_need_nothing++;
        return 0;
}

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, returning 1 if so.  This function is part of the
 * RCU implementation; it is -not- an exported member of the RCU API.
 */
static int rcu_pending(int cpu)
{
        struct rcu_state *rsp;

        for_each_rcu_flavor(rsp)
                if (__rcu_pending(rsp, per_cpu_ptr(rsp->rda, cpu)))
                        return 1;
        return 0;
}

/*
 * Return true if the specified CPU has any callback.  If all_lazy is
 * non-NULL, store an indication of whether all callbacks are lazy.
 * (If there are no callbacks, all of them are deemed to be lazy.)
 */
static int __maybe_unused rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
{
        bool al = true;
        bool hc = false;
        struct rcu_data *rdp;
        struct rcu_state *rsp;

        for_each_rcu_flavor(rsp) {
                rdp = per_cpu_ptr(rsp->rda, cpu);
                if (!rdp->nxtlist)
                        continue;
                hc = true;
                if (rdp->qlen != rdp->qlen_lazy || !all_lazy) {
                        al = false;
                        break;
                }
        }
        if (all_lazy)
                *all_lazy = al;
        return hc;
}

/*
 * Helper function for _rcu_barrier() tracing.  If tracing is disabled,
 * the compiler is expected to optimize this away.
 */
static void _rcu_barrier_trace(struct rcu_state *rsp, const char *s,
                               int cpu, unsigned long done)
{
        trace_rcu_barrier(rsp->name, s, cpu,
                          atomic_read(&rsp->barrier_cpu_count), done);
}

/*
 * RCU callback function for _rcu_barrier().  If we are last, wake
 * up the task executing _rcu_barrier().
 */
static void rcu_barrier_callback(struct rcu_head *rhp)
{
        struct rcu_data *rdp = container_of(rhp, struct rcu_data, barrier_head);
        struct rcu_state *rsp = rdp->rsp;

        if (atomic_dec_and_test(&rsp->barrier_cpu_count)) {
                _rcu_barrier_trace(rsp, "LastCB", -1, rsp->n_barrier_done);
                complete(&rsp->barrier_completion);
        } else {
                _rcu_barrier_trace(rsp, "CB", -1, rsp->n_barrier_done);
        }
}

/*
 * Called with preemption disabled, and from cross-cpu IRQ context.
 */
static void rcu_barrier_func(void *type)
{
        struct rcu_state *rsp = type;
        struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);

        _rcu_barrier_trace(rsp, "IRQ", -1, rsp->n_barrier_done);
        atomic_inc(&rsp->barrier_cpu_count);
        rsp->call(&rdp->barrier_head, rcu_barrier_callback);
}

/*
 * Orchestrate the specified type of RCU barrier, waiting for all
 * RCU callbacks of the specified type to complete.
 */
static void _rcu_barrier(struct rcu_state *rsp)
{
        int cpu;
        struct rcu_data *rdp;
        unsigned long snap = ACCESS_ONCE(rsp->n_barrier_done);
        unsigned long snap_done;

        _rcu_barrier_trace(rsp, "Begin", -1, snap);

        /* Take mutex to serialize concurrent rcu_barrier() requests. */
        mutex_lock(&rsp->barrier_mutex);

        /*
         * Ensure that all prior references, including to ->n_barrier_done,
         * are ordered before the _rcu_barrier() machinery.
         */
        smp_mb();  /* See above block comment. */

        /*
         * Recheck ->n_barrier_done to see if others did our work for us.
         * This means checking ->n_barrier_done for an even-to-odd-to-even
         * transition.  The "if" expression below therefore rounds the old
         * value up to the next even number and adds two before comparing.
         */
        snap_done = rsp->n_barrier_done;
        _rcu_barrier_trace(rsp, "Check", -1, snap_done);

        /*
         * If the value in snap is odd, we needed to wait for the current
         * rcu_barrier() to complete, then wait for the next one, in other
         * words, we need the value of snap_done to be three larger than
         * the value of snap.  On the other hand, if the value in snap is
         * even, we only had to wait for the next rcu_barrier() to complete,
         * in other words, we need the value of snap_done to be only two
         * greater than the value of snap.  The "(snap + 3) & ~0x1" computes
         * this for us (thank you, Linus!).
         */
        if (ULONG_CMP_GE(snap_done, (snap + 3) & ~0x1)) {
                _rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
                smp_mb(); /* caller's subsequent code after above check. */
                mutex_unlock(&rsp->barrier_mutex);
                return;
        }

        /*
         * Increment ->n_barrier_done to avoid duplicate work.  Use
         * ACCESS_ONCE() to prevent the compiler from speculating
         * the increment to precede the early-exit check.
         */
        ACCESS_ONCE(rsp->n_barrier_done)++;
        WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 1);
        _rcu_barrier_trace(rsp, "Inc1", -1, rsp->n_barrier_done);
        smp_mb(); /* Order ->n_barrier_done increment with below mechanism. */

        /*
         * Initialize the count to one rather than to zero in order to
         * avoid a too-soon return to zero in case of a short grace period
         * (or preemption of this task).  Exclude CPU-hotplug operations
         * to ensure that no offline CPU has callbacks queued.
         */
        init_completion(&rsp->barrier_completion);
        atomic_set(&rsp->barrier_cpu_count, 1);
        get_online_cpus();

        /*
         * Force each CPU with callbacks to register a new callback.
         * When that callback is invoked, we will know that all of the
         * corresponding CPU's preceding callbacks have been invoked.
         */
        for_each_possible_cpu(cpu) {
                if (!cpu_online(cpu) && !rcu_is_nocb_cpu(cpu))
                        continue;
                rdp = per_cpu_ptr(rsp->rda, cpu);
                if (rcu_is_nocb_cpu(cpu)) {
                        _rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
                                           rsp->n_barrier_done);
                        atomic_inc(&rsp->barrier_cpu_count);
                        __call_rcu(&rdp->barrier_head, rcu_barrier_callback,
                                   rsp, cpu, 0);
                } else if (ACCESS_ONCE(rdp->qlen)) {
                        _rcu_barrier_trace(rsp, "OnlineQ", cpu,
                                           rsp->n_barrier_done);
                        smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
                } else {
                        _rcu_barrier_trace(rsp, "OnlineNQ", cpu,
                                           rsp->n_barrier_done);
                }
        }
        put_online_cpus();

        /*
         * Now that we have an rcu_barrier_callback() callback on each
         * CPU, and thus each counted, remove the initial count.
         */
        if (atomic_dec_and_test(&rsp->barrier_cpu_count))
                complete(&rsp->barrier_completion);

        /* Increment ->n_barrier_done to prevent duplicate work. */
        smp_mb(); /* Keep increment after above mechanism. */
        ACCESS_ONCE(rsp->n_barrier_done)++;
        WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 0);
        _rcu_barrier_trace(rsp, "Inc2", -1, rsp->n_barrier_done);
        smp_mb(); /* Keep increment before caller's subsequent code. */

        /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
        wait_for_completion(&rsp->barrier_completion);

        /* Other rcu_barrier() invocations can now safely proceed. */
        mutex_unlock(&rsp->barrier_mutex);
}

/**
 * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
 */
void rcu_barrier_bh(void)
{
        _rcu_barrier(&rcu_bh_state);
}
EXPORT_SYMBOL_GPL(rcu_barrier_bh);

/**
 * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.
 */
void rcu_barrier_sched(void)
{
        _rcu_barrier(&rcu_sched_state);
}
EXPORT_SYMBOL_GPL(rcu_barrier_sched);

/*
 * Do boot-time initialization of a CPU's per-CPU RCU data.
 */
static void __init
rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
{
        unsigned long flags;
        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
        struct rcu_node *rnp = rcu_get_root(rsp);

        /* Set up local state, ensuring consistent view of global state. */
        raw_spin_lock_irqsave(&rnp->lock, flags);
        rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
        init_callback_list(rdp);
        rdp->qlen_lazy = 0;
        ACCESS_ONCE(rdp->qlen) = 0;
        rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
        WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
        WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
        rdp->cpu = cpu;
        rdp->rsp = rsp;
        rcu_boot_init_nocb_percpu_data(rdp);
        raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
 * Initialize a CPU's per-CPU RCU data.  Note that only one online or
 * offline event can be happening at a given time.  Note also that we
 * can accept some slop in the rsp->completed access due to the fact
 * that this CPU cannot possibly have any RCU callbacks in flight yet.
 */
static void
rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
{
        unsigned long flags;
        unsigned long mask;
        struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
        struct rcu_node *rnp = rcu_get_root(rsp);

        /* Exclude new grace periods. */
        mutex_lock(&rsp->onoff_mutex);

        /* Set up local state, ensuring consistent view of global state. */
        raw_spin_lock_irqsave(&rnp->lock, flags);
        rdp->beenonline = 1;     /* We have now been online. */
        rdp->qlen_last_fqs_check = 0;
        rdp->n_force_qs_snap = rsp->n_force_qs;
        rdp->blimit = blimit;
        init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
        rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
        rcu_sysidle_init_percpu_data(rdp->dynticks);
        atomic_set(&rdp->dynticks->dynticks,
                   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
        raw_spin_unlock(&rnp->lock);            /* irqs remain disabled. */

        /* Add CPU to rcu_node bitmasks. */
        rnp = rdp->mynode;
        mask = rdp->grpmask;
        do {
                /* Exclude any attempts to start a new GP on small systems. */
                raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
                rnp->qsmaskinit |= mask;
                mask = rnp->grpmask;
                if (rnp == rdp->mynode) {
                        /*
                         * If there is a grace period in progress, we will
                         * set up to wait for it next time we run the
                         * RCU core code.
                         */
                        rdp->gpnum = rnp->completed;
                        rdp->completed = rnp->completed;
                        rdp->passed_quiesce = 0;
                        rdp->qs_pending = 0;
                        trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuonl"));
                }
                raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
                rnp = rnp->parent;
        } while (rnp != NULL && !(rnp->qsmaskinit & mask));
        local_irq_restore(flags);

        mutex_unlock(&rsp->onoff_mutex);
}

static void rcu_prepare_cpu(int cpu)
{
        struct rcu_state *rsp;

        for_each_rcu_flavor(rsp)
                rcu_init_percpu_data(cpu, rsp);
}

/*
 * Handle CPU online/offline notification events.
 */
static int rcu_cpu_notify(struct notifier_block *self,
                                    unsigned long action, void *hcpu)
{
        long cpu = (long)hcpu;
        struct rcu_data *rdp = per_cpu_ptr(rcu_state_p->rda, cpu);
        struct rcu_node *rnp = rdp->mynode;
        struct rcu_state *rsp;

        trace_rcu_utilization(TPS("Start CPU hotplug"));
        switch (action) {
        case CPU_UP_PREPARE:
        case CPU_UP_PREPARE_FROZEN:
                rcu_prepare_cpu(cpu);
                rcu_prepare_kthreads(cpu);
                break;
        case CPU_ONLINE:
        case CPU_DOWN_FAILED:
                rcu_boost_kthread_setaffinity(rnp, -1);
                break;
        case CPU_DOWN_PREPARE:
                rcu_boost_kthread_setaffinity(rnp, cpu);
                break;
        case CPU_DYING:
        case CPU_DYING_FROZEN:
                for_each_rcu_flavor(rsp)
                        rcu_cleanup_dying_cpu(rsp);
                break;
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
        case CPU_UP_CANCELED:
        case CPU_UP_CANCELED_FROZEN:
                for_each_rcu_flavor(rsp)
                        rcu_cleanup_dead_cpu(cpu, rsp);
                break;
        default:
                break;
        }
        trace_rcu_utilization(TPS("End CPU hotplug"));
        return NOTIFY_OK;
}

static int rcu_pm_notify(struct notifier_block *self,
                         unsigned long action, void *hcpu)
{
        switch (action) {
        case PM_HIBERNATION_PREPARE:
        case PM_SUSPEND_PREPARE:
                if (nr_cpu_ids <= 256) /* Expediting bad for large systems. */
                        rcu_expedited = 1;
                break;
        case PM_POST_HIBERNATION:
        case PM_POST_SUSPEND:
                rcu_expedited = 0;
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

/*
 * Spawn the kthread that handles this RCU flavor's grace periods.
 */
static int __init rcu_spawn_gp_kthread(void)
{
        unsigned long flags;
        struct rcu_node *rnp;
        struct rcu_state *rsp;
        struct task_struct *t;

        for_each_rcu_flavor(rsp) {
                t = kthread_run(rcu_gp_kthread, rsp, "%s", rsp->name);
                BUG_ON(IS_ERR(t));
                rnp = rcu_get_root(rsp);
                raw_spin_lock_irqsave(&rnp->lock, flags);
                rsp->gp_kthread = t;
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                rcu_spawn_nocb_kthreads(rsp);
        }
        return 0;
}
early_initcall(rcu_spawn_gp_kthread);

/*
 * This function is invoked towards the end of the scheduler's initialization
 * process.  Before this is called, the idle task might contain
 * RCU read-side critical sections (during which time, this idle
 * task is booting the system).  After this function is called, the
 * idle tasks are prohibited from containing RCU read-side critical
 * sections.  This function also enables RCU lockdep checking.
 */
void rcu_scheduler_starting(void)
{
        WARN_ON(num_online_cpus() != 1);
        WARN_ON(nr_context_switches() > 0);
        rcu_scheduler_active = 1;
}

/*
 * Compute the per-level fanout, either using the exact fanout specified
 * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
 */
#ifdef CONFIG_RCU_FANOUT_EXACT
static void __init rcu_init_levelspread(struct rcu_state *rsp)
{
        int i;

        rsp->levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
        for (i = rcu_num_lvls - 2; i >= 0; i--)
                rsp->levelspread[i] = CONFIG_RCU_FANOUT;
}
#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
static void __init rcu_init_levelspread(struct rcu_state *rsp)
{
        int ccur;
        int cprv;
        int i;

        cprv = nr_cpu_ids;
        for (i = rcu_num_lvls - 1; i >= 0; i--) {
                ccur = rsp->levelcnt[i];
                rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
                cprv = ccur;
        }
}
#endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */

/*
 * Helper function for rcu_init() that initializes one rcu_state structure.
 */
static void __init rcu_init_one(struct rcu_state *rsp,
                struct rcu_data __percpu *rda)
{
        static char *buf[] = { "rcu_node_0",
                               "rcu_node_1",
                               "rcu_node_2",
                               "rcu_node_3" };  /* Match MAX_RCU_LVLS */
        static char *fqs[] = { "rcu_node_fqs_0",
                               "rcu_node_fqs_1",
                               "rcu_node_fqs_2",
                               "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
        static u8 fl_mask = 0x1;
        int cpustride = 1;
        int i;
        int j;
        struct rcu_node *rnp;

        BUILD_BUG_ON(MAX_RCU_LVLS > ARRAY_SIZE(buf));  /* Fix buf[] init! */

        /* Silence gcc 4.8 warning about array index out of range. */
        if (rcu_num_lvls > RCU_NUM_LVLS)
                panic("rcu_init_one: rcu_num_lvls overflow");

        /* Initialize the level-tracking arrays. */

        for (i = 0; i < rcu_num_lvls; i++)
                rsp->levelcnt[i] = num_rcu_lvl[i];
        for (i = 1; i < rcu_num_lvls; i++)
                rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
        rcu_init_levelspread(rsp);
        rsp->flavor_mask = fl_mask;
        fl_mask <<= 1;

        /* Initialize the elements themselves, starting from the leaves. */

        for (i = rcu_num_lvls - 1; i >= 0; i--) {
                cpustride *= rsp->levelspread[i];
                rnp = rsp->level[i];
                for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
                        raw_spin_lock_init(&rnp->lock);
                        lockdep_set_class_and_name(&rnp->lock,
                                                   &rcu_node_class[i], buf[i]);
                        raw_spin_lock_init(&rnp->fqslock);
                        lockdep_set_class_and_name(&rnp->fqslock,
                                                   &rcu_fqs_class[i], fqs[i]);
                        rnp->gpnum = rsp->gpnum;
                        rnp->completed = rsp->completed;
                        rnp->qsmask = 0;
                        rnp->qsmaskinit = 0;
                        rnp->grplo = j * cpustride;
                        rnp->grphi = (j + 1) * cpustride - 1;
                        if (rnp->grphi >= nr_cpu_ids)
                                rnp->grphi = nr_cpu_ids - 1;
                        if (i == 0) {
                                rnp->grpnum = 0;
                                rnp->grpmask = 0;
                                rnp->parent = NULL;
                        } else {
                                rnp->grpnum = j % rsp->levelspread[i - 1];
                                rnp->grpmask = 1UL << rnp->grpnum;
                                rnp->parent = rsp->level[i - 1] +
                                              j / rsp->levelspread[i - 1];
                        }
                        rnp->level = i;
                        INIT_LIST_HEAD(&rnp->blkd_tasks);
                        rcu_init_one_nocb(rnp);
                }
        }

        rsp->rda = rda;
        init_waitqueue_head(&rsp->gp_wq);
        rnp = rsp->level[rcu_num_lvls - 1];
        for_each_possible_cpu(i) {
                while (i > rnp->grphi)
                        rnp++;
                per_cpu_ptr(rsp->rda, i)->mynode = rnp;
                rcu_boot_init_percpu_data(i, rsp);
        }
        list_add(&rsp->flavors, &rcu_struct_flavors);
}

/*
 * Compute the rcu_node tree geometry from kernel parameters.  This cannot
 * replace the definitions in tree.h because those are needed to size
 * the ->node array in the rcu_state structure.
 */
static void __init rcu_init_geometry(void)
{
        ulong d;
        int i;
        int j;
        int n = nr_cpu_ids;
        int rcu_capacity[MAX_RCU_LVLS + 1];

        /*
         * Initialize any unspecified boot parameters.
         * The default values of jiffies_till_first_fqs and
         * jiffies_till_next_fqs are set to the RCU_JIFFIES_TILL_FORCE_QS
         * value, which is a function of HZ, then adding one for each
         * RCU_JIFFIES_FQS_DIV CPUs that might be on the system.
         */
        d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
        if (jiffies_till_first_fqs == ULONG_MAX)
                jiffies_till_first_fqs = d;
        if (jiffies_till_next_fqs == ULONG_MAX)
                jiffies_till_next_fqs = d;

        /* If the compile-time values are accurate, just leave. */
        if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
            nr_cpu_ids == NR_CPUS)
                return;
        pr_info("RCU: Adjusting geometry for rcu_fanout_leaf=%d, nr_cpu_ids=%d\n",
                rcu_fanout_leaf, nr_cpu_ids);

        /*
         * Compute number of nodes that can be handled an rcu_node tree
         * with the given number of levels.  Setting rcu_capacity[0] makes
         * some of the arithmetic easier.
         */
        rcu_capacity[0] = 1;
        rcu_capacity[1] = rcu_fanout_leaf;
        for (i = 2; i <= MAX_RCU_LVLS; i++)
                rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT;

        /*
         * The boot-time rcu_fanout_leaf parameter is only permitted
         * to increase the leaf-level fanout, not decrease it.  Of course,
         * the leaf-level fanout cannot exceed the number of bits in
         * the rcu_node masks.  Finally, the tree must be able to accommodate
         * the configured number of CPUs.  Complain and fall back to the
         * compile-time values if these limits are exceeded.
         */
        if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
            rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
            n > rcu_capacity[MAX_RCU_LVLS]) {
                WARN_ON(1);
                return;
        }

        /* Calculate the number of rcu_nodes at each level of the tree. */
        for (i = 1; i <= MAX_RCU_LVLS; i++)
                if (n <= rcu_capacity[i]) {
                        for (j = 0; j <= i; j++)
                                num_rcu_lvl[j] =
                                        DIV_ROUND_UP(n, rcu_capacity[i - j]);
                        rcu_num_lvls = i;
                        for (j = i + 1; j <= MAX_RCU_LVLS; j++)
                                num_rcu_lvl[j] = 0;
                        break;
                }

        /* Calculate the total number of rcu_node structures. */
        rcu_num_nodes = 0;
        for (i = 0; i <= MAX_RCU_LVLS; i++)
                rcu_num_nodes += num_rcu_lvl[i];
        rcu_num_nodes -= n;
}

void __init rcu_init(void)
{
        int cpu;

        rcu_bootup_announce();
        rcu_init_geometry();
        rcu_init_one(&rcu_bh_state, &rcu_bh_data);
        rcu_init_one(&rcu_sched_state, &rcu_sched_data);
        __rcu_init_preempt();
        open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

        /*
         * We don't need protection against CPU-hotplug here because
         * this is called early in boot, before either interrupts
         * or the scheduler are operational.
         */
        cpu_notifier(rcu_cpu_notify, 0);
        pm_notifier(rcu_pm_notify, 0);
        for_each_online_cpu(cpu)
                rcu_cpu_notify(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
}

#include "tree_plugin.h"