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    Subject[PATCH 3.10 040/139] workqueue: restore WQ_UNBOUND/max_active==1 to be ordered
    Date
    From: Tejun Heo <tj@kernel.org>

    commit 5c0338c68706be53b3dc472e4308961c36e4ece1 upstream.

    The combination of WQ_UNBOUND and max_active == 1 used to imply
    ordered execution. After NUMA affinity 4c16bd327c74 ("workqueue:
    implement NUMA affinity for unbound workqueues"), this is no longer
    true due to per-node worker pools.

    While the right way to create an ordered workqueue is
    alloc_ordered_workqueue(), the documentation has been misleading for a
    long time and people do use WQ_UNBOUND and max_active == 1 for ordered
    workqueues which can lead to subtle bugs which are very difficult to
    trigger.

    It's unlikely that we'd see noticeable performance impact by enforcing
    ordering on WQ_UNBOUND / max_active == 1 workqueues. Let's
    automatically set __WQ_ORDERED for those workqueues.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-by: Christoph Hellwig <hch@infradead.org>
    Reported-by: Alexei Potashnik <alexei@purestorage.com>
    Fixes: 4c16bd327c74 ("workqueue: implement NUMA affinity for unbound workqueues")
    Cc: stable@vger.kernel.org
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    kernel/workqueue.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    index 66972ac..3f8558f 100644
    --- a/kernel/workqueue.c
    +++ b/kernel/workqueue.c
    @@ -4213,6 +4213,16 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
    struct workqueue_struct *wq;
    struct pool_workqueue *pwq;

    + /*
    + * Unbound && max_active == 1 used to imply ordered, which is no
    + * longer the case on NUMA machines due to per-node pools. While
    + * alloc_ordered_workqueue() is the right way to create an ordered
    + * workqueue, keep the previous behavior to avoid subtle breakages
    + * on NUMA.
    + */
    + if ((flags & WQ_UNBOUND) && max_active == 1)
    + flags |= __WQ_ORDERED;
    +
    /* allocate wq and format name */
    if (flags & WQ_UNBOUND)
    tbl_size = wq_numa_tbl_len * sizeof(wq->numa_pwq_tbl[0]);
    --
    2.8.0.rc2.1.gbe9624a
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