From: Manfred Spraul Date: Tue, 11 Oct 2016 20:54:50 +0000 (-0700) Subject: ipc/sem.c: fix complex_count vs. simple op race X-Git-Tag: v4.9-rc1~42^2~24 X-Git-Url: http://git.cascardo.info/?p=cascardo%2Flinux.git;a=commitdiff_plain;h=5864a2fd3088db73d47942370d0f7210a807b9bc ipc/sem.c: fix complex_count vs. simple op race Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a race: sem_lock has a fast path that allows parallel simple operations. There are two reasons why a simple operation cannot run in parallel: - a non-simple operations is ongoing (sma->sem_perm.lock held) - a complex operation is sleeping (sma->complex_count != 0) As both facts are stored independently, a thread can bypass the current checks by sleeping in the right positions. See below for more details (or kernel bugzilla 105651). The patch fixes that by creating one variable (complex_mode) that tracks both reasons why parallel operations are not possible. The patch also updates stale documentation regarding the locking. With regards to stable kernels: The patch is required for all kernels that include the commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") (3.10?) The alternative is to revert the patch that introduced the race. The patch is safe for backporting, i.e. it makes no assumptions about memory barriers in spin_unlock_wait(). Background: Here is the race of the current implementation: Thread A: (simple op) - does the first "sma->complex_count == 0" test Thread B: (complex op) - does sem_lock(): This includes an array scan. But the scan can't find Thread A, because Thread A does not own sem->lock yet. - the thread does the operation, increases complex_count, drops sem_lock, sleeps Thread A: - spin_lock(&sem->lock), spin_is_locked(sma->sem_perm.lock) - sleeps before the complex_count test Thread C: (complex op) - does sem_lock (no array scan, complex_count==1) - wakes up Thread B. - decrements complex_count Thread A: - does the complex_count test Bug: Now both thread A and thread C operate on the same array, without any synchronization. Fixes: 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") Link: http://lkml.kernel.org/r/1469123695-5661-1-git-send-email-manfred@colorfullife.com Reported-by: Cc: "H. Peter Anvin" Cc: Peter Zijlstra Cc: Davidlohr Bueso Cc: Thomas Gleixner Cc: Ingo Molnar Cc: <1vier1@web.de> Cc: [3.10+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/include/linux/sem.h b/include/linux/sem.h index 976ce3a19f1b..d0efd6e6c20a 100644 --- a/include/linux/sem.h +++ b/include/linux/sem.h @@ -21,6 +21,7 @@ struct sem_array { struct list_head list_id; /* undo requests on this array */ int sem_nsems; /* no. of semaphores in array */ int complex_count; /* pending complex operations */ + bool complex_mode; /* no parallel simple ops */ }; #ifdef CONFIG_SYSVIPC diff --git a/ipc/sem.c b/ipc/sem.c index 7c9d4f7683c0..5e318c5f749d 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -162,14 +162,21 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it); /* * Locking: + * a) global sem_lock() for read/write * sem_undo.id_next, * sem_array.complex_count, - * sem_array.pending{_alter,_cont}, - * sem_array.sem_undo: global sem_lock() for read/write - * sem_undo.proc_next: only "current" is allowed to read/write that field. + * sem_array.complex_mode + * sem_array.pending{_alter,_const}, + * sem_array.sem_undo * + * b) global or semaphore sem_lock() for read/write: * sem_array.sem_base[i].pending_{const,alter}: - * global or semaphore sem_lock() for read/write + * sem_array.complex_mode (for read) + * + * c) special: + * sem_undo_list.list_proc: + * * undo_list->lock for write + * * rcu for read */ #define sc_semmsl sem_ctls[0] @@ -260,30 +267,61 @@ static void sem_rcu_free(struct rcu_head *head) } /* - * Wait until all currently ongoing simple ops have completed. + * Enter the mode suitable for non-simple operations: * Caller must own sem_perm.lock. - * New simple ops cannot start, because simple ops first check - * that sem_perm.lock is free. - * that a) sem_perm.lock is free and b) complex_count is 0. */ -static void sem_wait_array(struct sem_array *sma) +static void complexmode_enter(struct sem_array *sma) { int i; struct sem *sem; - if (sma->complex_count) { - /* The thread that increased sma->complex_count waited on - * all sem->lock locks. Thus we don't need to wait again. - */ + if (sma->complex_mode) { + /* We are already in complex_mode. Nothing to do */ return; } + /* We need a full barrier after seting complex_mode: + * The write to complex_mode must be visible + * before we read the first sem->lock spinlock state. + */ + smp_store_mb(sma->complex_mode, true); + for (i = 0; i < sma->sem_nsems; i++) { sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } + /* + * spin_unlock_wait() is not a memory barriers, it is only a + * control barrier. The code must pair with spin_unlock(&sem->lock), + * thus just the control barrier is insufficient. + * + * smp_rmb() is sufficient, as writes cannot pass the control barrier. + */ + smp_rmb(); +} + +/* + * Try to leave the mode that disallows simple operations: + * Caller must own sem_perm.lock. + */ +static void complexmode_tryleave(struct sem_array *sma) +{ + if (sma->complex_count) { + /* Complex ops are sleeping. + * We must stay in complex mode + */ + return; + } + /* + * Immediately after setting complex_mode to false, + * a simple op can start. Thus: all memory writes + * performed by the current operation must be visible + * before we set complex_mode to false. + */ + smp_store_release(&sma->complex_mode, false); } +#define SEM_GLOBAL_LOCK (-1) /* * If the request contains only one semaphore operation, and there are * no complex transactions pending, lock only the semaphore involved. @@ -300,56 +338,42 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, /* Complex operation - acquire a full lock */ ipc_lock_object(&sma->sem_perm); - /* And wait until all simple ops that are processed - * right now have dropped their locks. - */ - sem_wait_array(sma); - return -1; + /* Prevent parallel simple ops */ + complexmode_enter(sma); + return SEM_GLOBAL_LOCK; } /* * Only one semaphore affected - try to optimize locking. - * The rules are: - * - optimized locking is possible if no complex operation - * is either enqueued or processed right now. - * - The test for enqueued complex ops is simple: - * sma->complex_count != 0 - * - Testing for complex ops that are processed right now is - * a bit more difficult. Complex ops acquire the full lock - * and first wait that the running simple ops have completed. - * (see above) - * Thus: If we own a simple lock and the global lock is free - * and complex_count is now 0, then it will stay 0 and - * thus just locking sem->lock is sufficient. + * Optimized locking is possible if no complex operation + * is either enqueued or processed right now. + * + * Both facts are tracked by complex_mode. */ sem = sma->sem_base + sops->sem_num; - if (sma->complex_count == 0) { + /* + * Initial check for complex_mode. Just an optimization, + * no locking, no memory barrier. + */ + if (!sma->complex_mode) { /* * It appears that no complex operation is around. * Acquire the per-semaphore lock. */ spin_lock(&sem->lock); - /* Then check that the global lock is free */ - if (!spin_is_locked(&sma->sem_perm.lock)) { - /* - * We need a memory barrier with acquire semantics, - * otherwise we can race with another thread that does: - * complex_count++; - * spin_unlock(sem_perm.lock); - */ - smp_acquire__after_ctrl_dep(); + /* + * See 51d7d5205d33 + * ("powerpc: Add smp_mb() to arch_spin_is_locked()"): + * A full barrier is required: the write of sem->lock + * must be visible before the read is executed + */ + smp_mb(); - /* - * Now repeat the test of complex_count: - * It can't change anymore until we drop sem->lock. - * Thus: if is now 0, then it will stay 0. - */ - if (sma->complex_count == 0) { - /* fast path successful! */ - return sops->sem_num; - } + if (!smp_load_acquire(&sma->complex_mode)) { + /* fast path successful! */ + return sops->sem_num; } spin_unlock(&sem->lock); } @@ -369,15 +393,16 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, /* Not a false alarm, thus complete the sequence for a * full lock. */ - sem_wait_array(sma); - return -1; + complexmode_enter(sma); + return SEM_GLOBAL_LOCK; } } static inline void sem_unlock(struct sem_array *sma, int locknum) { - if (locknum == -1) { + if (locknum == SEM_GLOBAL_LOCK) { unmerge_queues(sma); + complexmode_tryleave(sma); ipc_unlock_object(&sma->sem_perm); } else { struct sem *sem = sma->sem_base + locknum; @@ -529,6 +554,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params) } sma->complex_count = 0; + sma->complex_mode = true; /* dropped by sem_unlock below */ INIT_LIST_HEAD(&sma->pending_alter); INIT_LIST_HEAD(&sma->pending_const); INIT_LIST_HEAD(&sma->list_id); @@ -2184,10 +2210,10 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it) /* * The proc interface isn't aware of sem_lock(), it calls * ipc_lock_object() directly (in sysvipc_find_ipc). - * In order to stay compatible with sem_lock(), we must wait until - * all simple semop() calls have left their critical regions. + * In order to stay compatible with sem_lock(), we must + * enter / leave complex_mode. */ - sem_wait_array(sma); + complexmode_enter(sma); sem_otime = get_semotime(sma); @@ -2204,6 +2230,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it) sem_otime, sma->sem_ctime); + complexmode_tryleave(sma); + return 0; } #endif