linux-kernel - [ 052/105] cgroup: notify_on_release may not be triggered in some cases

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Message-Id: <20121028231553.071436947@decadent.org.uk>
Date:	Sun, 28 Oct 2012 23:16:28 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Ben Blum <bblum@...rew.cmu.edu>, Li Zefan <lizefan@...wei.com>,
	Tejun Heo <tj@...nel.org>
Subject: [ 052/105] cgroup: notify_on_release may not be triggered in some cases

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>

commit 1f5320d5972aa50d3e8d2b227b636b370e608359 upstream.

notify_on_release must be triggered when the last process in a cgroup is
move to another. But if the first(and only) process in a cgroup is moved to
another, notify_on_release is not triggered.

	# mkdir /cgroup/cpu/SRC
	# mkdir /cgroup/cpu/DST
	#
	# echo 1 >/cgroup/cpu/SRC/notify_on_release
	# echo 1 >/cgroup/cpu/DST/notify_on_release
	#
	# sleep 300 &
	[1] 8629
	#
	# echo 8629 >/cgroup/cpu/SRC/tasks
	# echo 8629 >/cgroup/cpu/DST/tasks
	-> notify_on_release for /SRC must be triggered at this point,
	   but it isn't.

This is because put_css_set() is called before setting CGRP_RELEASABLE
in cgroup_task_migrate(), and is a regression introduce by the
commit:74a1166d(cgroups: make procs file writable), which was merged
into v3.0.

Cc: Ben Blum <bblum@...rew.cmu.edu>
Acked-by: Li Zefan <lizefan@...wei.com>
Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Signed-off-by: Tejun Heo <tj@...nel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/cgroup.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1803,9 +1803,8 @@ static int cgroup_task_migrate(struct cg
 	 * trading it for newcg is protected by cgroup_mutex, we're safe to drop
 	 * it here; it will be freed under RCU.
 	 */
-	put_css_set(oldcg);
-
 	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
+	put_css_set(oldcg);
 	return 0;
 }
 


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