From mboxrd@z Thu Jan 1 00:00:00 1970 From: Frederic Weisbecker Subject: Re: [PATCH 2/2 v2] cgroup: Drop task_lock(parent) on cgroup_fork() Date: Wed, 21 Dec 2011 03:48:52 +0100 Message-ID: <20111221024849.GC17668__36957.3717508048$1324435749$gmane$org@somewhere> References: <1324432958-20414-1-git-send-email-fweisbec@gmail.com> <1324432958-20414-3-git-send-email-fweisbec@gmail.com> <4EF14176.9040206@cn.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <4EF14176.9040206-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org> 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: Li Zefan Cc: Containers , LKML , Oleg Nesterov , Mandeep Singh Baines , Tejun Heo , Cgroups , Andrew Morton , Paul Menage List-Id: containers.vger.kernel.org On Wed, Dec 21, 2011 at 10:16:22AM +0800, Li Zefan wrote: > Frederic Weisbecker wrote: > > We don't need to hold the parent task_lock() on the > > parent in cgroup_fork() because we are already synchronized > > against the two places that may change the parent css_set > > concurrently: > > > > - cgroup_exit(), but the parent obviously can't exit concurrently > > - cgroup migration: we are synchronized against threadgroup_lock() > > > > So we can safely remove the task_lock() there. > > > > Signed-off-by: Frederic Weisbecker > > Cc: Tejun Heo > > Cc: Li Zefan > > Cc: Containers > > Cc: Cgroups > > Cc: KAMEZAWA Hiroyuki > > Cc: Oleg Nesterov > > Cc: Andrew Morton > > Cc: Paul Menage > > Cc: Mandeep Singh Baines > > --- > > kernel/cgroup.c | 10 +++++++--- > > 1 files changed, 7 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c > > index 24f6d6f..1999f60 100644 > > --- a/kernel/cgroup.c > > +++ b/kernel/cgroup.c > > @@ -4556,7 +4556,7 @@ static const struct file_operations proc_cgroupstats_operations = { > > * > > * A pointer to the shared css_set was automatically copied in > > * fork.c by dup_task_struct(). However, we ignore that copy, since > > - * it was not made under the protection of RCU or cgroup_mutex, so > > + * it was not made under the protection of threadgroup_change_begin(), so > > I think the original comment still stands, but now threadgroup_change_begin() > can also protect the cgroup pointer from becoming invalid. Right but I'm not sure it's worth quoting RCU and cgroup_mutex. The reason why we use threadgroup_change_begin() is not only to ensure the pointer validity but also to synchronize the whole cgroup proc logic. This way when we attach a whole proc with cgroup_attach_proc(), we are sure that no thread forked too soon or too late such that it wouldn't be migrated with the rest. RCU or cgroup_mutex on dup_task_struct() (+ a get_css_set()) would have protected the pointer validity but not the whole above described machinery. So I don't think it's even worth quoting those solutions. But if you prefer I can keep the old comment. OTOH what I think is missing in the comment is that explanation on the synchronization against entire proc migration. I can edit that. > > > * might no longer be a valid cgroup pointer. cgroup_attach_task() might > > * have already changed current->cgroups, allowing the previously > > * referenced cgroup group to be removed and freed. > > @@ -4566,10 +4566,14 @@ static const struct file_operations proc_cgroupstats_operations = { > > */ > > void cgroup_fork(struct task_struct *child) > > { > > - task_lock(current); > > + /* > > + * We don't need to task_lock() current because current->cgroups > > + * can't be changed concurrently here. The parent obviously hasn't > > + * exited and called cgroup_exit(), and we are synchronized against > > + * cgroup migration through threadgroup_change_begin(). > > + */ > > child->cgroups = current->cgroups; > > get_css_set(child->cgroups); > > - task_unlock(current); > > INIT_LIST_HEAD(&child->cg_list); > > } > >