[3.8.y.z extended stable] Patch "cgroup: update cgroup_enable_task_cg_lists() to grab siglock" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    cgroup: update cgroup_enable_task_cg_lists() to grab siglock

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.20.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From cd4f26ef20c8e09de7a1e8e38346d593b3541993 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj at kernel.org>
Date: Thu, 13 Feb 2014 13:29:31 -0500
Subject: cgroup: update cgroup_enable_task_cg_lists() to grab siglock

commit 532de3fc72adc2a6525c4d53c07bf81e1732083d upstream.

Currently, there's nothing preventing cgroup_enable_task_cg_lists()
from missing set PF_EXITING and race against cgroup_exit().  Depending
on the timing, cgroup_exit() may finish with the task still linked on
css_set leading to list corruption.  Fix it by grabbing siglock in
cgroup_enable_task_cg_lists() so that PF_EXITING is guaranteed to be
visible.

This whole on-demand cg_list optimization is extremely fragile and has
ample possibility to lead to bugs which can cause things like
once-a-year oops during boot.  I'm wondering whether the better
approach would be just adding "cgroup_disable=all" handling which
disables the whole cgroup rather than tempting fate with this
on-demand craziness.

Signed-off-by: Tejun Heo <tj at kernel.org>
Acked-by: Li Zefan <lizefan at huawei.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/cgroup.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ba9e28c..a2d2c80 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2982,9 +2982,14 @@ static void cgroup_enable_task_cg_lists(void)
 		 * We should check if the process is exiting, otherwise
 		 * it will race with cgroup_exit() in that the list
 		 * entry won't be deleted though the process has exited.
+		 * Do it while holding siglock so that we don't end up
+		 * racing against cgroup_exit().
 		 */
+		spin_lock_irq(&p->sighand->siglock);
 		if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
 			list_add(&p->cg_list, &p->cgroups->tasks);
+		spin_unlock_irq(&p->sighand->siglock);
+
 		task_unlock(p);
 	} while_each_thread(g, p);
 	read_unlock(&tasklist_lock);
--
1.8.3.2