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Subject[PATCH 22/52] cgroups: fix 2.6.32 regression causing BUG_ON() in cgroup_diput()
From: Dave Anderson <>

commit bd4f490a079730aadfaf9a728303ea0135c01945 upstream.

The LTP cgroup test suite generates a "kernel BUG at kernel/cgroup.c:790!"
here in cgroup_diput():

* if we're getting rid of the cgroup, refcount should ensure
* that there are no pidlists left.

The cgroup pidlist rework in 2.6.32 generates the BUG_ON, which is caused
when pidlist_array_load() calls cgroup_pidlist_find():

(1) if a matching cgroup_pidlist is found, it down_write's the mutex of the
pre-existing cgroup_pidlist, and increments its use_count.
(2) if no matching cgroup_pidlist is found, then a new one is allocated, it
down_write's its mutex, and the use_count is set to 0.
(3) the matching, or new, cgroup_pidlist gets returned back to pidlist_array_load(),
which increments its use_count -- regardless whether new or pre-existing --
and up_write's the mutex.

So if a matching list is ever encountered by cgroup_pidlist_find() during
the life of a cgroup directory, it results in an inflated use_count value,
preventing it from ever getting released by cgroup_release_pid_array().
Then if the directory is subsequently removed, cgroup_diput() hits the
BUG_ON() when it finds that the directory's cgroup is still populated with
a pidlist.

The patch simply removes the use_count increment when a matching pidlist
is found by cgroup_pidlist_find(), because it gets bumped by the calling
pidlist_array_load() function while still protected by the list's mutex.

Signed-off-by: Dave Anderson <>
Reviewed-by: Li Zefan <>
Acked-by: Ben Blum <>
Cc: Paul Menage <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>
kernel/cgroup.c | 1 -
1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0249f4b..1fbcc74 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2468,7 +2468,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
/* make sure l doesn't vanish out from under us */
- l->use_count++;
return l;

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