[3.8.y.z extended stable] Patch "cgroup: fix locking in cgroup_cfts_commit()" 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: fix locking in cgroup_cfts_commit()

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 160b9ab45807e730e8032cc143fbc128385ece82 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj at kernel.org>
Date: Sat, 8 Feb 2014 10:26:34 -0500
Subject: cgroup: fix locking in cgroup_cfts_commit()

commit 48573a893303986e3b0b2974d6fb11f3d1bb7064 upstream.

cgroup_cfts_commit() walks the cgroup hierarchy that the target
subsystem is attached to and tries to apply the file changes.  Due to
the convolution with inode locking, it can't keep cgroup_mutex locked
while iterating.  It currently holds only RCU read lock around the
actual iteration and then pins the found cgroup using dget().

Unfortunately, this is incorrect.  Although the iteration does check
cgroup_is_dead() before invoking dget(), there's nothing which
prevents the dentry from going away inbetween.  Note that this is
different from the usual css iterations where css_tryget() is used to
pin the css - css_tryget() tests whether the css can be pinned and
fails if not.

The problem can be solved by simply holding cgroup_mutex instead of
RCU read lock around the iteration, which actually reduces LOC.

Signed-off-by: Tejun Heo <tj at kernel.org>
Acked-by: Li Zefan <lizefan at huawei.com>
[ kamal: backport to 3.8 (context) ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/cgroup.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 754b917..ba9e28c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2826,8 +2826,6 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 		sb = NULL;
 	}

-	mutex_unlock(&cgroup_mutex);
-
 	/*
 	 * All new cgroups will see @cfts update on @ss->cftsets.  Add/rm
 	 * files for all cgroups which were created before.
@@ -2835,16 +2833,17 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 	list_for_each_entry_safe(cgrp, n, &pending, cft_q_node) {
 		struct inode *inode = cgrp->dentry->d_inode;

+		mutex_unlock(&cgroup_mutex);
 		mutex_lock(&inode->i_mutex);
 		mutex_lock(&cgroup_mutex);
 		if (!cgroup_is_removed(cgrp))
 			cgroup_addrm_files(cgrp, ss, cfts, is_add);
-		mutex_unlock(&cgroup_mutex);
 		mutex_unlock(&inode->i_mutex);

 		list_del_init(&cgrp->cft_q_node);
 		dput(cgrp->dentry);
 	}
+	mutex_unlock(&cgroup_mutex);

 	if (sb)
 		deactivate_super(sb);
--
1.8.3.2