From mboxrd@z Thu Jan 1 00:00:00 1970 From: Li Zefan Subject: Re: [PATCH 2/2 v2] cgroup: Drop task_lock(parent) on cgroup_fork() Date: Wed, 21 Dec 2011 10:16:22 +0800 Message-ID: <4EF14176.9040206__34003.0447849587$1324433671$gmane$org@cn.fujitsu.com> References: <1324432958-20414-1-git-send-email-fweisbec@gmail.com> <1324432958-20414-3-git-send-email-fweisbec@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1324432958-20414-3-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@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: Frederic Weisbecker Cc: Containers , LKML , Oleg Nesterov , Mandeep Singh Baines , Tejun Heo , Cgroups , Andrew Morton , Paul Menage List-Id: containers.vger.kernel.org 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. > * 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); > } >