From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] memcg: Only free spare array when readers are done Date: Thu, 14 Jan 2016 15:38:47 +0100 Message-ID: <20160114143847.GD5046@dhcp22.suse.cz> References: <001a113abaa499606605294b5b17@google.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <001a113abaa499606605294b5b17-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Martijn Coenen Cc: Johannes Weiner , Vladimir Davydov , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Thu 14-01-16 14:33:52, Martijn Coenen wrote: > A spare array holding mem cgroup threshold events is kept around > to make sure we can always safely deregister an event and have an > array to store the new set of events in. > > In the scenario where we're going from 1 to 0 registered events, the > pointer to the primary array containing 1 event is copied to the spare > slot, and then the spare slot is freed because no events are left. > However, it is freed before calling synchronize_rcu(), which means > readers may still be accessing threshold->primary after it is freed. Have you seen this triggering in the real life? > > Fixed by only freeing after synchronize_rcu(). > Fixes: 8c7577637ca3 ("memcg: free spare array to avoid memory leak") > Signed-off-by: Martijn Coenen Cc: stable Acked-by: Michal Hocko Thanks! > --- > mm/memcontrol.c | 11 ++++++----- > 1 file changed, 6 insertions(+), 5 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 14cb1db..73228b6 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3522,16 +3522,17 @@ static void > __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg, > swap_buffers: > /* Swap primary and spare array */ > thresholds->spare = thresholds->primary; > - /* If all events are unregistered, free the spare array */ > - if (!new) { > - kfree(thresholds->spare); > - thresholds->spare = NULL; > - } > > rcu_assign_pointer(thresholds->primary, new); > > /* To be sure that nobody uses thresholds */ > synchronize_rcu(); > + > + /* If all events are unregistered, free the spare array */ > + if (!new) { > + kfree(thresholds->spare); > + thresholds->spare = NULL; > + } > unlock: > mutex_unlock(&memcg->thresholds_lock); > } > -- > 2.6.0.rc2.230.g3dd15c0 -- Michal Hocko SUSE Labs