Messages in this thread Patch in this message | | | Date | Wed, 23 Dec 2009 13:48:42 -0500 | From | Dave Anderson <> | Subject | [PATCH] cgroups: fix 2.6.32 regression causing BUG_ON() in cgroup_diput() |
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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. */ BUG_ON(!list_empty(&cgrp->pidlists));
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 <anderson@redhat.com>
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--- linux-2.6-git/kernel/cgroup.c.orig +++ linux-2.6-git/kernel/cgroup.c @@ -2468,7 +2468,6 @@ static struct cgroup_pidlist *cgroup_pid /* make sure l doesn't vanish out from under us */ down_write(&l->mutex); mutex_unlock(&cgrp->pidlist_mutex); - l->use_count++; return l; } }
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