arm64: factor work_pending state machine to C

Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 441420c..6a64182 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -707,18 +707,13 @@ ret_fast_syscall_trace:
  * Ok, we need to do extra processing, enter the slow path.
  */
 work_pending:
-       tbnz    x1, #TIF_NEED_RESCHED, work_resched
-       /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
        mov     x0, sp                          // 'regs'
-       enable_irq                              // enable interrupts for do_notify_resume()
        bl      do_notify_resume
-       b       ret_to_user
-work_resched:
 #ifdef CONFIG_TRACE_IRQFLAGS
-       bl      trace_hardirqs_off              // the IRQs are off here, inform the tracing code
+       bl      trace_hardirqs_on               // enabled while in userspace
 #endif
-       bl      schedule
-
+       ldr     x1, [tsk, #TI_FLAGS]            // re-check for single-step
+       b       finish_ret_to_user
 /*
  * "slow" syscall return path.
  */
@@ -727,6 +722,7 @@ ret_to_user:
        ldr     x1, [tsk, #TI_FLAGS]
        and     x2, x1, #_TIF_WORK_MASK
        cbnz    x2, work_pending
+finish_ret_to_user:
        enable_step_tsk x1, x2
        kernel_exit 0
 ENDPROC(ret_to_user)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index a8eafdb..404dd67 100644 (file)
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -402,15 +402,31 @@ static void do_signal(struct pt_regs *regs)
 asmlinkage void do_notify_resume(struct pt_regs *regs,
                                 unsigned int thread_flags)
 {
-       if (thread_flags & _TIF_SIGPENDING)
-               do_signal(regs);
-
-       if (thread_flags & _TIF_NOTIFY_RESUME) {
-               clear_thread_flag(TIF_NOTIFY_RESUME);
-               tracehook_notify_resume(regs);
-       }
-
-       if (thread_flags & _TIF_FOREIGN_FPSTATE)
-               fpsimd_restore_current_state();
+       /*
+        * The assembly code enters us with IRQs off, but it hasn't
+        * informed the tracing code of that for efficiency reasons.
+        * Update the trace code with the current status.
+        */
+       trace_hardirqs_off();
+       do {
+               if (thread_flags & _TIF_NEED_RESCHED) {
+                       schedule();
+               } else {
+                       local_irq_enable();
+
+                       if (thread_flags & _TIF_SIGPENDING)
+                               do_signal(regs);
+
+                       if (thread_flags & _TIF_NOTIFY_RESUME) {
+                               clear_thread_flag(TIF_NOTIFY_RESUME);
+                               tracehook_notify_resume(regs);
+                       }
+
+                       if (thread_flags & _TIF_FOREIGN_FPSTATE)
+                               fpsimd_restore_current_state();
+               }
 
+               local_irq_disable();
+               thread_flags = READ_ONCE(current_thread_info()->flags);
+       } while (thread_flags & _TIF_WORK_MASK);
 }