[PATCH 3.13 085/163] perf: Fix a race condition in perf_remove_from_context()

[Kamal Mostafa]

3.13.11.9 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Cong Wang <cwang at twopensource.com>

commit 3577af70a2ce4853d58e57d832e687d739281479 upstream.

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 at twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong at gmail.com>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
Cc: Paul Mackerras <paulus at samba.org>
Cc: Arnaldo Carvalho de Melo <acme at kernel.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Link: http://lkml.kernel.org/r/1409696840-843-1-git-send-email-xiyou.wangcong@gmail.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/events/core.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f8519d6..38f0d1b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1503,6 +1503,11 @@ 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;
 	}
 
@@ -1942,6 +1947,11 @@ 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;
 	}
 
-- 
1.9.1