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Commit a0507c84 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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perf: Annotate perf_event_read_group() vs perf_event_release_kernel() 



Stephane reported a lockdep warning while using PERF_FORMAT_GROUP.

The issue is that perf_event_read_group() takes faults while holding
the ctx->mutex, while perf_event_release_kernel() can be called from
munmap(). Which makes for an AB-BA deadlock.

Except we can never establish the deadlock because we'll only ever
call perf_event_release_kernel() after all file descriptors are dead
so there is no concurrency possible.

Reported-by: default avatarStephane Eranian <eranian@google.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent cce91317

kernel/perf_event.c

+14 −2
Original line number Original line Diff line number Diff line
@@ -1867,7 +1867,19 @@ int perf_event_release_kernel(struct perf_event *event) 
	event->state = PERF_EVENT_STATE_FREE;

	event->state = PERF_EVENT_STATE_FREE;





	WARN_ON_ONCE(ctx->parent_ctx);

	WARN_ON_ONCE(ctx->parent_ctx);

	mutex_lock(&ctx->mutex);

	/*

	 * There are two ways this annotation is useful:

	 *

	 *  1) there is a lock recursion from perf_event_exit_task

	 *     see the comment there.

	 *

	 *  2) there is a lock-inversion with mmap_sem through

	 *     perf_event_read_group(), which takes faults while

	 *     holding ctx->mutex, however this is called after

	 *     the last filedesc died, so there is no possibility

	 *     to trigger the AB-BA case.

	 */

	mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);

	perf_event_remove_from_context(event);

	perf_event_remove_from_context(event);

	mutex_unlock(&ctx->mutex);

	mutex_unlock(&ctx->mutex);
@@ -5305,7 +5317,7 @@ void perf_event_exit_task(struct task_struct *child) 
	 *

	 *

	 * But since its the parent context it won't be the same instance.

	 * But since its the parent context it won't be the same instance.

	 */

	 */

	mutex_lock_nested(&child_ctx->mutex, SINGLE_DEPTH_NESTING);

	mutex_lock(&child_ctx->mutex);





again:

again:

	list_for_each_entry_safe(child_event, tmp, &child_ctx->pinned_groups,

	list_for_each_entry_safe(child_event, tmp, &child_ctx->pinned_groups,