On Mon, Oct 08, 2012 at 11:00:00AM +0900, Tejun Heo wrote:

7e381b0eb1 ("cgroup: Drop task_lock(parent) on cgroup_fork()") removed
task_lock from cgroup_fork citing that current->cgroups can't change
due to threadgroup_change locking; however, threadgroup_change locking
is used only during CLONE_THREAD forking.  If @current is forking a
new process, there's nothing preventing someone else to migrate the
parent while forking is in progress and delete the css_set it
currently is using.  Am I confused somewhere?

Also, please note that task_lock is likely to be hot on local CPU at
that point and avoiding it there might not really buy much.

Thanks.

-- 
tejun