Commit 7e3aa30a authored Dec 23, 2011 by Frederic Weisbecker Committed by Tejun Heo Dec 27, 2011

cgroup: Remove task_lock() from cgroup_post_fork()



cgroup_post_fork() is protected between threadgroup_change_begin()
and threadgroup_change_end() against concurrent changes of the
child's css_set in cgroup_task_migrate(). Also the child can't
exit and call cgroup_exit() at this stage, this means it's css_set
can't be changed with init_css_set concurrently.

For these reasons, we don't need to hold task_lock() on the child
because it's css_set can only remain stable in this place.

Let's remove the lock there.

v2: Update comment to explain that we are safe against
cgroup_exit()

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <cgroups@vger.kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Mandeep Singh Baines <msb@chromium.org>

parent c6ca5750

kernel/cgroup.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -4595,10 +4595,19 @@ void cgroup_post_fork(struct task_struct *child)
		{
		if (use_task_css_set_links) {
		write_lock(&css_set_lock);
		task_lock(child);
		if (list_empty(&child->cg_list))
		if (list_empty(&child->cg_list)) {
		/*
		* It's safe to use child->cgroups without task_lock()
		* here because we are protected through
		* threadgroup_change_begin() against concurrent
		* css_set change in cgroup_task_migrate(). Also
		* the task can't exit at that point until
		* wake_up_new_task() is called, so we are protected
		* against cgroup_exit() setting child->cgroup to
		* init_css_set.
		*/
		list_add(&child->cg_list, &child->cgroups->tasks);
		task_unlock(child);
		}
		write_unlock(&css_set_lock);
		}
		}