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

[Luis Henriques]

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-3.5.y-queue branch of the 3.5.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-3.5.y-queue

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

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 7fb2d59a31a3c26a63b3a9d566290fd9a6df3247 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>
[ luis: backported to 3.5: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/cgroup.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7720f52..e4a16ed 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2941,9 +2941,14 @@ 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);
--
1.9.0