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cgroup: Remove task_lock() from cgroup_post_fork()

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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>
This commit is contained in:
Frederic Weisbecker 2011-12-23 04:25:23 +01:00 committed by Tejun Heo
parent c6ca57500c
commit 7e3aa30ac8
1 changed files with 12 additions and 3 deletions
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15
kernel/cgroup.c
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@ -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);
}
}