From mboxrd@z Thu Jan 1 00:00:00 1970 From: Glauber Costa Subject: Re: [PATCH] fix bad behavior in use_hierarchy file Date: Mon, 25 Jun 2012 16:11:01 +0400 Message-ID: <4FE85555.1010209@parallels.com> References: <1340616061-1955-1-git-send-email-glommer@parallels.com> <20120625120823.GK19805@tiehlicka.suse.cz> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120625120823.GK19805-VqjxzfR4DlwKmadIfiO5sKVXKuFTiq87@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Michal Hocko Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Andrew Morton , devel-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org, Dhaval Giani , Kamezawa Hiroyuki , Johannes Weiner On 06/25/2012 04:08 PM, Michal Hocko wrote: > On Mon 25-06-12 13:21:01, Glauber Costa wrote: >> I have an application that does the following: >> >> * copy the state of all controllers attached to a hierarchy >> * replicate it as a child of the current level. >> >> I would expect writes to the files to mostly succeed, since they >> are inheriting sane values from parents. >> >> But that is not the case for use_hierarchy. If it is set to 0, we >> succeed ok. If we're set to 1, the value of the file is automatically >> set to 1 in the children, but if userspace tries to write the >> very same 1, it will fail. That same situation happens if we >> set use_hierarchy, create a child, and then try to write 1 again. >> >> Now, there is no reason whatsoever for failing to write a value >> that is already there. It doesn't even match the comments, that >> states: >> >> /* If parent's use_hierarchy is set, we can't make any modifications >> * in the child subtrees... >> >> since we are not changing anything. >> >> The following patch tests the new value against the one we're storing, >> and automatically return 0 if we're not proposing a change. > > Fair enough. > >> >> Signed-off-by: Glauber Costa >> CC: Dhaval Giani >> CC: Michal Hocko >> CC: Kamezawa Hiroyuki >> CC: Johannes Weiner > > One comment bellow... > Acked-by: Michal Hocko > >> --- >> mm/memcontrol.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index ac35bcc..cccebbc 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -3779,6 +3779,10 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft, >> parent_memcg = mem_cgroup_from_cont(parent); >> >> cgroup_lock(); >> + >> + if (memcg->use_hierarchy == val) >> + goto out; >> + > > Why do you need cgroup_lock to check the value? Even if we have 2 > CPUs racing (one trying to set to 0 other to 1 with use_hierarchy==0) > then the "set to 0" operation might fail depending on who hits the > cgroup_lock first anyway. > > So while this is correct I think there is not much point to take the global > cgroup lock in this case. > Well, no. All operations will succeed, unless the cgroup breeds new children. That's the operation we're racing against. So we need to guarantee a snapshot of what is the status of the file in the moment we said we'd create a new children. Besides, I believe taking the lock is conceptually the right thing to do, even if by an ordering artifact we would happen to be safe.