Commit 3577af70 authored Sep 02, 2014 by Cong Wang Committed by Ingo Molnar Sep 09, 2014

perf: Fix a race condition in perf_remove_from_context()



We saw a kernel soft lockup in perf_remove_from_context(),
it looks like the `perf` process, when exiting, could not go
out of the retry loop. Meanwhile, the target process was forking
a child. So either the target process should execute the smp
function call to deactive the event (if it was running) or it should
do a context switch which deactives the event.

It seems we optimize out a context switch in perf_event_context_sched_out(),
and what's more important, we still test an obsolete task pointer when
retrying, so no one actually would deactive that event in this situation.
Fix it directly by reloading the task pointer in perf_remove_from_context().

This should cure the above soft lockup.

Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/1409696840-843-1-git-send-email-xiyou.wangcong@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 256aae5e

kernel/events/core.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -1524,6 +1524,11 @@ retry:
		*/
		if (ctx->is_active) {
		raw_spin_unlock_irq(&ctx->lock);
		/*
		* Reload the task pointer, it might have been changed by
		* a concurrent perf_event_context_sched_out().
		*/
		task = ctx->task;
		goto retry;
		}

		@@ -1967,6 +1972,11 @@ retry:
		*/
		if (ctx->is_active) {
		raw_spin_unlock_irq(&ctx->lock);
		/*
		* Reload the task pointer, it might have been changed by
		* a concurrent perf_event_context_sched_out().
		*/
		task = ctx->task;
		goto retry;
		}

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