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* [PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork()
@ 2011-12-23  3:25 Frederic Weisbecker
       [not found] ` <1324610723-30080-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
  0 siblings, 1 reply; 4+ messages in thread
From: Frederic Weisbecker @ 2011-12-23  3:25 UTC (permalink / raw)
  To: Tejun Heo, Li Zefan
  Cc: LKML, Frederic Weisbecker, Containers, Cgroups, KAMEZAWA Hiroyuki,
	Oleg Nesterov, Andrew Morton, Paul Menage, Mandeep Singh Baines

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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@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 |   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

^ permalink raw reply related	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2011-12-27 17:06 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2011-12-23  3:25 [PATCH v2] cgroup: Remove task_lock() from cgroup_post_fork() Frederic Weisbecker
     [not found] ` <1324610723-30080-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2011-12-23 23:00   ` Tejun Heo
     [not found]     ` <20111223230013.GV17084-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
2011-12-27  1:33       ` Li Zefan
2011-12-27 17:06   ` Tejun Heo

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