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From: tip-bot for Peter Zijlstra on 7 May 2010 14:50
Commit-ID: a0507c84bf47dfd204299774f45fd16da33f0619
Gitweb: http://git.kernel.org/tip/a0507c84bf47dfd204299774f45fd16da33f0619
Author: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
AuthorDate: Thu, 6 May 2010 15:42:53 +0200
Committer: Ingo Molnar <mingo(a)elte.hu>
CommitDate: Fri, 7 May 2010 11:30:59 +0200

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: Stephane Eranian <eranian(a)google.com>
Cc: Paul Mackerras <paulus(a)samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo(a)elte.hu>
---
kernel/perf_event.c | 16 ++++++++++++++--
1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 49d8be5..34659d4 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1867,7 +1867,19 @@ int perf_event_release_kernel(struct perf_event *event)
event->state = PERF_EVENT_STATE_FREE;

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);
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.
*/
- mutex_lock_nested(&child_ctx->mutex, SINGLE_DEPTH_NESTING);
+ mutex_lock(&child_ctx->mutex);

again:
list_for_each_entry_safe(child_event, tmp, &child_ctx->pinned_groups,
--
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Next: [PATCH 0/1] perf list documentation changes