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 <redacted>
Cc: Li Zefan <redacted>
Cc: Tejun Heo <tj@kernel.org>
Cc: Containers <redacted>
Cc: Cgroups <redacted>
Cc: KAMEZAWA Hiroyuki <redacted>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <redacted>
Cc: Mandeep Singh Baines <redacted>
---
 kernel/cgroup.c |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1042b3c..0ccfc25 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -4593,10 +4593,19 @@ 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);
 	}
 }

-- 
1.7.5.4