Commit 3577af70 authored by Cong Wang's avatar Cong Wang Committed by Ingo Molnar

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: default avatarCong Wang <cwang@twopensource.com>
Signed-off-by: default avatarCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: default avatarPeter 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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 256aae5e
...@@ -1524,6 +1524,11 @@ static void perf_remove_from_context(struct perf_event *event, bool detach_group ...@@ -1524,6 +1524,11 @@ static void perf_remove_from_context(struct perf_event *event, bool detach_group
*/ */
if (ctx->is_active) { if (ctx->is_active) {
raw_spin_unlock_irq(&ctx->lock); 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; goto retry;
} }
...@@ -1967,6 +1972,11 @@ perf_install_in_context(struct perf_event_context *ctx, ...@@ -1967,6 +1972,11 @@ perf_install_in_context(struct perf_event_context *ctx,
*/ */
if (ctx->is_active) { if (ctx->is_active) {
raw_spin_unlock_irq(&ctx->lock); 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; goto retry;
} }
......
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