[PATCH 2/2] cgroup: Drop task_lock(parent) on cgroup_fork()

[Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

We don't need to hold the parent task_lock() on the
parent in cgroup_fork() because we are already synchronized
against the two places that may change the parent css_set
concurrently:

- cgroup_exit(), but the parent obviously can't exit concurrently
- cgroup migration: we are synchronized against threadgroup_lock()

So we can safely remove the task_lock() there.

Signed-off-by: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
---
 kernel/cgroup.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6b5f693..a1e87a4 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4544,7 +4544,7 @@ static const struct file_operations proc_cgroupstats_operations = {
  *
  * A pointer to the shared css_set was automatically copied in
  * fork.c by dup_task_struct().  However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
+ * it was not made under the protection of threadgroup_change_begin(), so
  * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
  * have already changed current->cgroups, allowing the previously
  * referenced cgroup group to be removed and freed.
@@ -4554,10 +4554,14 @@ static const struct file_operations proc_cgroupstats_operations = {
  */
 void cgroup_fork(struct task_struct *child)
 {
-	task_lock(current);
+	/*
+	 * We don't need to task_lock() current because current->cgroups
+	 * can't be changed concurrently here. The parent obviously hasn't
+	 * exited and called cgroup_exit(), and we are synchronized against
+	 * cgroup migration through threadgroup_change_begin().
+	 */
 	child->cgroups = current->cgroups;
 	get_css_set(child->cgroups);
-	task_unlock(current);
 	INIT_LIST_HEAD(&child->cg_list);
 }
 
-- 
1.7.5.4