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    Subject[PATCH 3.13.y-ckt 077/138] workqueue: handle NUMA_NO_NODE for unbound pool_workqueue lookup
    Date
    3.13.11-ckt36 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Tejun Heo <tj@kernel.org>

    commit d6e022f1d207a161cd88e08ef0371554680ffc46 upstream.

    When looking up the pool_workqueue to use for an unbound workqueue,
    workqueue assumes that the target CPU is always bound to a valid NUMA
    node. However, currently, when a CPU goes offline, the mapping is
    destroyed and cpu_to_node() returns NUMA_NO_NODE.

    This has always been broken but hasn't triggered often enough before
    874bbfe600a6 ("workqueue: make sure delayed work run in local cpu").
    After the commit, workqueue forcifully assigns the local CPU for
    delayed work items without explicit target CPU to fix a different
    issue. This widens the window where CPU can go offline while a
    delayed work item is pending causing delayed work items dispatched
    with target CPU set to an already offlined CPU. The resulting
    NUMA_NO_NODE mapping makes workqueue try to queue the work item on a
    NULL pool_workqueue and thus crash.

    While 874bbfe600a6 has been reverted for a different reason making the
    bug less visible again, it can still happen. Fix it by mapping
    NUMA_NO_NODE to the default pool_workqueue from unbound_pwq_by_node().
    This is a temporary workaround. The long term solution is keeping CPU
    -> NODE mapping stable across CPU off/online cycles which is being
    worked on.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
    Cc: Tang Chen <tangchen@cn.fujitsu.com>
    Cc: Rafael J. Wysocki <rafael@kernel.org>
    Cc: Len Brown <len.brown@intel.com>
    Link: http://lkml.kernel.org/g/1454424264.11183.46.camel@gmail.com
    Link: http://lkml.kernel.org/g/1453702100-2597-1-git-send-email-tangchen@cn.fujitsu.com
    [ luis: backported to 3.16: adjusted context ]
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    kernel/workqueue.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    index eb1cc22..de5ad5c 100644
    --- a/kernel/workqueue.c
    +++ b/kernel/workqueue.c
    @@ -558,6 +558,16 @@ static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
    int node)
    {
    assert_rcu_or_wq_mutex(wq);
    +
    + /*
    + * XXX: @node can be NUMA_NO_NODE if CPU goes offline while a
    + * delayed item is pending. The plan is to keep CPU -> NODE
    + * mapping valid and stable across CPU on/offlines. Once that
    + * happens, this workaround can be removed.
    + */
    + if (unlikely(node == NUMA_NO_NODE))
    + return wq->dfl_pwq;
    +
    return rcu_dereference_raw(wq->numa_pwq_tbl[node]);
    }

    --
    2.7.0
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