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    Subject[PATCH 3/2] fix flush_workqueue() vs CPU_DEAD race
    "[PATCH 1/2] reimplement flush_workqueue()" fixed one race when CPU goes down
    while flush_cpu_workqueue() plays with it. But there is another problem, CPU
    can die before flush_workqueue() has a chance to call flush_cpu_workqueue().
    In that case pending work_structs can migrate to CPU which was already checked,
    so we should redo the "for_each_online_cpu(cpu)" loop.

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

    --- mm-6.20-rc2/kernel/workqueue.c~3_race 2006-12-29 18:37:31.000000000 +0300
    +++ mm-6.20-rc2/kernel/workqueue.c 2006-12-30 18:09:07.000000000 +0300
    @@ -65,6 +65,7 @@ struct workqueue_struct {

    /* All the per-cpu workqueues on the system, for hotplug cpu to add/remove
    threads to each one as cpus come/go. */
    +static long hotplug_sequence __read_mostly;
    static DEFINE_MUTEX(workqueue_mutex);
    static LIST_HEAD(workqueues);

    @@ -454,10 +455,16 @@ void fastcall flush_workqueue(struct wor
    /* Always use first cpu's area. */
    flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
    } else {
    + long sequence;
    int cpu;
    +again:
    + sequence = hotplug_sequence;

    for_each_online_cpu(cpu)
    flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
    +
    + if (unlikely(sequence != hotplug_sequence))
    + goto again;
    }
    mutex_unlock(&workqueue_mutex);
    }
    @@ -874,6 +881,7 @@ static int __devinit workqueue_cpu_callb
    cleanup_workqueue_thread(wq, hotcpu);
    list_for_each_entry(wq, &workqueues, list)
    take_over_work(wq, hotcpu);
    + hotplug_sequence++;
    break;

    case CPU_LOCK_RELEASE:
    -
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