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    Subject[PATCH 2/3] workqueues: implement flush_work()
    Most of users of flush_workqueue() can be changed to use cancel_work_sync(),
    but sometimes we really need to wait for the completion and cancelling is not
    an option. schedule_on_each_cpu() is good example.

    Add the new helper, flush_work(work), which waits for the completion of the
    specific work_struct. More precisely, it "flushes" the result of of the last
    queue_work() which is visible to the caller.

    For example, this code

    queue_work(wq, work);
    /* WINDOW */
    queue_work(wq, work);

    flush_work(work);

    doesn't necessary work "as expected". What can happen in the WINDOW above is

    - wq starts the execution of work->func()

    - the caller migrates to another CPU

    now, after the 2nd queue_work() this work is active on the previous CPU, and
    at the same time it is queued on another. In this case flush_work(work) may
    return before the first work->func() completes.

    It is trivial to add another helper

    int flush_work_sync(struct work_struct *work)
    {
    return flush_work(work) || wait_on_work(work);
    }

    which works "more correctly", but it has to iterate over all CPUs and thus
    it much slower than flush_work().

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
    Acked-By: Max Krasnyansky <maxk@qualcomm.com>

    --- 26-rc2/include/linux/workqueue.h~WQ_2_FLUSH_WORK 2008-05-18 15:42:34.000000000 +0400
    +++ 26-rc2/include/linux/workqueue.h 2008-05-18 15:42:34.000000000 +0400
    @@ -198,6 +198,8 @@ extern int keventd_up(void);
    extern void init_workqueues(void);
    int execute_in_process_context(work_func_t fn, struct execute_work *);

    +extern int flush_work(struct work_struct *work);
    +
    extern int cancel_work_sync(struct work_struct *work);

    /*
    --- 26-rc2/kernel/workqueue.c~WQ_2_FLUSH_WORK 2008-06-12 21:28:13.000000000 +0400
    +++ 26-rc2/kernel/workqueue.c 2008-06-29 18:20:33.000000000 +0400
    @@ -399,6 +399,52 @@ void flush_workqueue(struct workqueue_st
    }
    EXPORT_SYMBOL_GPL(flush_workqueue);

    +/**
    + * flush_work - block until a work_struct's callback has terminated
    + * @work: the work which is to be flushed
    + *
    + * It is expected that, prior to calling flush_work(), the caller has
    + * arranged for the work to not be requeued, otherwise it doesn't make
    + * sense to use this function.
    + */
    +int flush_work(struct work_struct *work)
    +{
    + struct cpu_workqueue_struct *cwq;
    + struct list_head *prev;
    + struct wq_barrier barr;
    +
    + might_sleep();
    + cwq = get_wq_data(work);
    + if (!cwq)
    + return 0;
    +
    + prev = NULL;
    + spin_lock_irq(&cwq->lock);
    + if (!list_empty(&work->entry)) {
    + /*
    + * See the comment near try_to_grab_pending()->smp_rmb().
    + * If it was re-queued under us we are not going to wait.
    + */
    + smp_rmb();
    + if (unlikely(cwq != get_wq_data(work)))
    + goto out;
    + prev = &work->entry;
    + } else {
    + if (cwq->current_work != work)
    + goto out;
    + prev = &cwq->worklist;
    + }
    + insert_wq_barrier(cwq, &barr, prev->next);
    +out:
    + spin_unlock_irq(&cwq->lock);
    + if (!prev)
    + return 0;
    +
    + wait_for_completion(&barr.done);
    + return 1;
    +}
    +EXPORT_SYMBOL_GPL(flush_work);
    +
    /*
    * Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,
    * so this work can't be re-armed in any way.


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