linux-kernel - Re: [PATCH] perf: Synchronously cleanup child events

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Date:	Mon, 18 Jan 2016 14:37:06 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH] perf: Synchronously cleanup child events

Peter Zijlstra <peterz@...radead.org> writes:

> I suspect this races against inherit_event(), like:
>
> 	inherit_event()			perf_event_release_kernel()
>
> 	if (is_orphaned_event(parent_event) /* false */
>
> 					event->owner = NULL
>
> 	mutex_lock(child_mutex);
> 	list_splice
> 	mutex_unlock(child_mutex);
>
> 					mutex_lock(child_mutex);
> 					list_add_tail
> 					mutex_unlock(child_mutex);

Or how about this instead:

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8eb3fee429..cd9f1ac537 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3786,8 +3786,9 @@ int perf_event_release_kernel(struct perf_event *event)
 
 	event->owner = NULL;
 
+retry:
 	/*
-	 * event::child_mutex nests inside ctx::lock, so move children
+	 * event::child_mutex nests inside ctx::mutex, so move children
 	 * to a safe place first and avoid inversion
 	 */
 	mutex_lock(&event->child_mutex);
@@ -3818,8 +3819,13 @@ int perf_event_release_kernel(struct perf_event *event)
 		put_event(event);
 	}
 
-	/* Must be the last reference */
+	/* Must be the last reference, .. */
 	put_event(event);
+
+	/* .. unless we raced with inherit_event(), in which case, repeat */
+	if (atomic_long_inc_not_zero(&event->refcount))
+		goto retry;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(perf_event_release_kernel);