Revert "cgroup: Remove task_lock() from cgroup_post_fork()"

This reverts commit 7e3aa30ac8c904a706518b725c451bb486daaae9.

The commit incorrectly assumed that fork path always performed
threadgroup_change_begin/end() and depended on that for
synchronization against task exit and cgroup migration paths instead
of explicitly grabbing task_lock().

threadgroup_change is not locked when forking a new process (as
opposed to a new thread in the same process) and even if it were it
wouldn't be effective as different processes use different threadgroup
locks.

Revert the incorrect optimization.

Signed-off-by: Tejun Heo <tj@kernel.org>
LKML-Reference: <20121008020000.GB2575@localhost>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: stable@vger.kernel.org

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 75aec12..f24f724 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4883,19 +4883,10 @@ void cgroup_post_fork(struct task_struct *child)
         */
        if (use_task_css_set_links) {
                write_lock(&css_set_lock);
-               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.
-                        */
+               task_lock(child);
+               if (list_empty(&child->cg_list))
                        list_add(&child->cg_list, &child->cgroups->tasks);
-               }
+               task_unlock(child);
                write_unlock(&css_set_lock);
        }
 }