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SubjectRe: [PATCH 20/43] workqueue: reimplement work flushing using linked works
On 02/26, Tejun Heo wrote:
>
> +static void move_linked_works(struct work_struct *work, struct list_head *head,
> + struct work_struct **nextp)
> +{
> ...
> + work = list_entry(work->entry.prev, struct work_struct, entry);
> + list_for_each_entry_safe_continue(work, n, NULL, entry) {

list_for_each_entry_safe_from(work) ? It doesn't need to move this
work back.

> @@ -680,7 +734,27 @@ static int worker_thread(void *__worker)
> if (kthread_should_stop())
> break;
>
> - run_workqueue(worker);
> + spin_lock_irq(&cwq->lock);
> +
> + while (!list_empty(&cwq->worklist)) {
> + struct work_struct *work =
> + list_first_entry(&cwq->worklist,
> + struct work_struct, entry);
> +
> + if (likely(!(*work_data_bits(work) &
> + WORK_STRUCT_LINKED))) {
> + /* optimization path, not strictly necessary */
> + process_one_work(worker, work);
> + if (unlikely(!list_empty(&worker->scheduled)))
> + process_scheduled_works(worker);
> + } else {
> + move_linked_works(work, &worker->scheduled,
> + NULL);
> + process_scheduled_works(worker);
> + }
> + }

So. If the next pending work W doesn't have WORK_STRUCT_LINKED,
it will be executed first, then we flush ->scheduled.

But,

> static void insert_wq_barrier(struct cpu_workqueue_struct *cwq,
> - struct wq_barrier *barr, struct list_head *head)
> + struct wq_barrier *barr,
> + struct work_struct *target, struct worker *worker)
> {
> ...
> + /*
> + * If @target is currently being executed, schedule the
> + * barrier to the worker; otherwise, put it after @target.
> + */
> + if (worker)
> + head = worker->scheduled.next;

this is the "target == current_work" case,

> - insert_work(cwq, &barr->work, head, work_color_to_flags(WORK_NO_COLOR));
> + insert_work(cwq, &barr->work, head,
> + work_color_to_flags(WORK_NO_COLOR) | linked);
> }

and in this case we put this barrier at the head of ->scheduled list.

This means, this barrier will run after that work W, not before it?


Hmm. And what if there are no pending works but ->current_work == target ?
Again, we add the barrier to ->scheduled, but in this case worker_thread()
can't even notice ->scheduled is not empty because it only checks ->worklist?

Well. most probably I just misread this code completely...


insert_wq_barrier() also does:

unsigned long *bits = work_data_bits(target);
...
*bits |= WORK_STRUCT_LINKED;

perhaps this needs atomic_long_set(), although I am not sure this really
matters.

Oleg.



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