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Subject[patch 19/94] cgroups: fix a race between cgroup_clone and umount
2.6.28-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Li Zefan <lizf@cn.fujitsu.com>

commit 7b574b7b0124ed344911f5d581e9bc2d83bbeb19 upstream.

The race is calling cgroup_clone() while umounting the ns cgroup subsys,
and thus cgroup_clone() might access invalid cgroup_fs, or kill_sb() is
called after cgroup_clone() created a new dir in it.

The BUG I triggered is BUG_ON(root->number_of_cgroups != 1);

------------[ cut here ]------------
kernel BUG at kernel/cgroup.c:1093!
invalid opcode: 0000 [#1] SMP
...
Process umount (pid: 5177, ti=e411e000 task=e40c4670 task.ti=e411e000)
...
Call Trace:
[<c0493df7>] ? deactivate_super+0x3f/0x51
[<c04a3600>] ? mntput_no_expire+0xb3/0xdd
[<c04a3ab2>] ? sys_umount+0x265/0x2ac
[<c04a3b06>] ? sys_oldumount+0xd/0xf
[<c0403911>] ? sysenter_do_call+0x12/0x31
...
EIP: [<c0456e76>] cgroup_kill_sb+0x23/0xe0 SS:ESP 0068:e411ef2c
---[ end trace c766c1be3bf944ac ]---
Cc: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/cgroup.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2942,7 +2942,11 @@ int cgroup_clone(struct task_struct *tsk
parent = task_cgroup(tsk, subsys->subsys_id);

/* Pin the hierarchy */
- atomic_inc(&parent->root->sb->s_active);
+ if (!atomic_inc_not_zero(&parent->root->sb->s_active)) {
+ /* We race with the final deactivate_super() */
+ mutex_unlock(&cgroup_mutex);
+ return 0;
+ }

/* Keep the cgroup alive */
get_css_set(cg);


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