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    Subject[PATCH 3.13 006/149] memcg: fix endless loop in __mem_cgroup_iter_next()
    Date
    3.13-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Hugh Dickins <hughd@google.com>

    commit ce48225fe3b1b0d1fc9fceb96ac3d8a879e45114 upstream.

    Commit 0eef615665ed ("memcg: fix css reference leak and endless loop in
    mem_cgroup_iter") got the interaction with the commit a few before it
    d8ad30559715 ("mm/memcg: iteration skip memcgs not yet fully
    initialized") slightly wrong, and we didn't notice at the time.

    It's elusive, and harder to get than the original, but for a couple of
    days before rc1, I several times saw a endless loop similar to that
    supposedly being fixed.

    This time it was a tighter loop in __mem_cgroup_iter_next(): because we
    can get here when our root has already been offlined, and the ordering
    of conditions was such that we then just cycled around forever.

    Fixes: 0eef615665ed ("memcg: fix css reference leak and endless loop in mem_cgroup_iter").
    Signed-off-by: Hugh Dickins <hughd@google.com>
    Acked-by: Michal Hocko <mhocko@suse.cz>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Greg Thelen <gthelen@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@linuxfoundation.org>

    ---
    mm/memcontrol.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -1108,8 +1108,8 @@ skip_node:
    * skipping css reference should be safe.
    */
    if (next_css) {
    - if ((next_css->flags & CSS_ONLINE) &&
    - (next_css == &root->css || css_tryget(next_css)))
    + if ((next_css == &root->css) ||
    + ((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
    return mem_cgroup_from_css(next_css);

    prev_css = next_css;



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