[PATCH] CGroups: Fix a race between rmdir and remount

[Paul Menage <menage@google.com>]

Fix a race between rmdir and remount

When a cgroup is removed, it's unlinked from its parent's children
list, but not actually freed until the last dentry on it is released
(at which point cgrp->root->number_of_cgroups is decremented).

Currently rebind_subsystems checks for the top cgroup's child list
being empty in order to rebind subsystems into or out of a hierarchy -
this can result in the set of subsystems bound to a hierarchy being
different than the set of subsystems with state in the
removed-but-not-freed cgroup.

The simplest fix for this is to forbid remounts that change the set of
subsystems on a hierarchy that has removed-but-not-freed cgroups.

This bug can be reproduced via:

mkdir /mnt/cg
mount -t cgroup -o ns,freezer cgroup /mnt/cg
mkdir /mnt/cg/foo
sleep 1h < /mnt/cg/foo &
rmdir /mnt/cg/foo
mount -t cgroup -o remount,ns,devices,freezer cgroup /mnt/cg
kill $!

Signed-off-by: Paul Menage <menage@google.com>

---
 kernel/cgroup.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
===================================================================
--- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c
+++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
@@ -702,7 +702,7 @@ static int rebind_subsystems(struct cgro
 	 * any child cgroups exist. This is theoretically supportable
 	 * but involves complex error handling, so it's being left until
 	 * later */
-	if (!list_empty(&cgrp->children))
+	if (root->number_of_cgroups > 1)
 		return -EBUSY;

 	/* Process each subsystem */