linux-kernel - [ 30/95] mm: memcg: Correct unregistring of events attached to the same eventfd

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Message-Id: <20120309190204.769938863@linuxfoundation.org>
Date:	Fri, 09 Mar 2012 11:02:32 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Michal Hocko <mhocko@...e.cz>
Subject: [ 30/95] mm: memcg: Correct unregistring of events attached to the same eventfd

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

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

From: Anton Vorontsov <anton.vorontsov@...aro.org>

commit 371528caec553785c37f73fa3926ea0de84f986f upstream.

There is an issue when memcg unregisters events that were attached to
the same eventfd:

- On the first call mem_cgroup_usage_unregister_event() removes all
  events attached to a given eventfd, and if there were no events left,
  thresholds->primary would become NULL;

- Since there were several events registered, cgroups core will call
  mem_cgroup_usage_unregister_event() again, but now kernel will oops,
  as the function doesn't expect that threshold->primary may be NULL.

That's a good question whether mem_cgroup_usage_unregister_event()
should actually remove all events in one go, but nowadays it can't
do any better as cftype->unregister_event callback doesn't pass
any private event-associated cookie. So, let's fix the issue by
simply checking for threshold->primary.

FWIW, w/o the patch the following oops may be observed:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
 IP: [<ffffffff810be32c>] mem_cgroup_usage_unregister_event+0x9c/0x1f0
 Pid: 574, comm: kworker/0:2 Not tainted 3.3.0-rc4+ #9 Bochs Bochs
 RIP: 0010:[<ffffffff810be32c>]  [<ffffffff810be32c>] mem_cgroup_usage_unregister_event+0x9c/0x1f0
 RSP: 0018:ffff88001d0b9d60  EFLAGS: 00010246
 Process kworker/0:2 (pid: 574, threadinfo ffff88001d0b8000, task ffff88001de91cc0)
 Call Trace:
  [<ffffffff8107092b>] cgroup_event_remove+0x2b/0x60
  [<ffffffff8103db94>] process_one_work+0x174/0x450
  [<ffffffff8103e413>] worker_thread+0x123/0x2d0

Signed-off-by: Anton Vorontsov <anton.vorontsov@...aro.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Kirill A. Shutemov <kirill@...temov.name>
Cc: Michal Hocko <mhocko@...e.cz>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/memcontrol.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4502,6 +4502,9 @@ static void mem_cgroup_usage_unregister_
 	 */
 	BUG_ON(!thresholds);
 
+	if (!thresholds->primary)
+		goto unlock;
+
 	usage = mem_cgroup_usage(memcg, type == _MEMSWAP);
 
 	/* Check if a threshold crossed before removing */
@@ -4550,7 +4553,7 @@ swap_buffers:
 
 	/* To be sure that nobody uses thresholds */
 	synchronize_rcu();
-
+unlock:
 	mutex_unlock(&memcg->thresholds_lock);
 }
 


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