From mboxrd@z Thu Jan 1 00:00:00 1970 From: Li Zefan Subject: Re: [PATCH 4/8] cgroup: deactivate CSS's and mark cgroup dead before invoking ->pre_destroy() Date: Mon, 5 Nov 2012 13:37:47 +0800 Message-ID: <509750AB.7070905@huawei.com> References: <1351712650-23709-1-git-send-email-tj@kernel.org> <1351712650-23709-5-git-send-email-tj@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1351712650-23709-5-git-send-email-tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Tejun Heo Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, mhocko-AlSwsSmVLrQ@public.gmane.org, hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org On 2012/11/1 3:44, Tejun Heo wrote: > Because ->pre_destroy() could fail and can't be called under > cgroup_mutex, cgroup destruction did something very ugly. > > 1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise. > > 2. Release cgroup_mutex and call ->pre_destroy(). > > 3. Re-grab cgroup_mutex and verify it can still be destroyed; fail > otherwise. > > 4. Continue destroying. > > In addition to being ugly, it has been always broken in various ways. > For example, memcg ->pre_destroy() expects the cgroup to be inactive > after it's done but tasks can be attached and detached between #2 and > #3 and the conditions that memcg verified in ->pre_destroy() might no > longer hold by the time control reaches #3. > > Now that ->pre_destroy() is no longer allowed to fail. We can switch > to the following. > > 1. Grab cgroup_mutex and verify it can be destroyed; fail otherwise. > > 2. Deactivate CSS's and mark the cgroup removed thus preventing any > further operations which can invalidate the verification from #1. > > 3. Release cgroup_mutex and call ->pre_destroy(). > > 4. Re-grab cgroup_mutex and continue destroying. > > After this change, controllers can safely assume that ->pre_destroy() > will only be called only once for a given cgroup and, once > ->pre_destroy() is called, the cgroup will stay dormant till it's > destroyed. > > This removes the only reason ->pre_destroy() can fail - new task being > attached or child cgroup being created inbetween. Error out path is > removed and ->pre_destroy() invocation is open coded in > cgroup_rmdir(). > > v2: cgroup_call_pre_destroy() removal moved to this patch per Michal. > Commit message updated per Glauber. > > Signed-off-by: Tejun Heo > Reviewed-by: Michal Hocko > Cc: Glauber Costa Acked-by: Li Zefan