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

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