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    Subject[PATCH 22/52] cgroups: fix 2.6.32 regression causing BUG_ON() in cgroup_diput()
    Date
    From: Dave Anderson <anderson@redhat.com>

    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.
    */
    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>
    Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
    Acked-by: Ben Blum <bblum@andrew.cmu.edu>
    Cc: Paul Menage <menage@google.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 | 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 */
    down_write(&l->mutex);
    mutex_unlock(&cgrp->pidlist_mutex);
    - l->use_count++;
    return l;
    }
    }
    --
    1.6.6


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