Hello, Lai.
On Mon, Sep 17, 2012 at 10:40:05AM +0800, Lai Jiangshan wrote:
> try_to_grab_pending() leave LINKED tagalong in delayed queue when
> it deletes a work. This behavior will cause future
> cwq_activate_first_delayed() increase the ->nr_active wrongly,
> and may cause the whole cwq frozen.
>
> example:
>
> state: cwq->max_active = 1, cwq->nr_active = 1
> one work in cwq->pool, many in cwq->delayed_works.
>
> step1: try_to_grab_pending() remove a work from delayed_works but
> leave tagalong.
> step2: when the work in cwq->pool is finished,
> cwq_activate_first_delayed()
> move the tagalong to cwq->pool and increase the ->nr_active.
>
> current state: cwq->nr_active = 1, but works of the cwq in cwq->pool are all
> NO_COLOR,
> so even when these works are finished, cwq->nr_active will not be
> decreased,
> and no work will be moved from cwq->delayed_works. the cwq is frozen.
>
> Fix it by moving the tagalong to cwq->pool in try_to_grab_pending().
>
> Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
Nice spotting.
> @@ -1101,11 +1101,26 @@ static int try_to_grab_pending(struct work_struct
> *work, bool is_dwork,
> */
> smp_rmb();
> if (gcwq == get_work_gcwq(work)) {
> + unsigned long delayed;
> +
> + /*
> + * move LINKED tagalong(if exist) out from
> + * delayed queue. Otherwise some future
> + * cwq_activate_first_delayed() may move
> + * works(which are all NO_COLOR) to cwq->pool
> + * and increase the ->nr_active. It may
> + * cause the whole cwq frozen.
> + */
> + delayed = *work_data_bits(work) & WORK_STRUCT_DELAYED;
> + if (delayed)
> + move_linked_works(work,
> + &get_work_cwq(work)->pool->worklist,
> + NULL);
> +
> debug_work_deactivate(work);
> list_del_init(&work->entry);
> cwq_dec_nr_in_flight(get_work_cwq(work),
> - get_work_color(work),
> - *work_data_bits(work) & WORK_STRUCT_DELAYED);
> + get_work_color(work), delayed);
I'm a bit uneasy about how this introduces something which is on
active list but not quite active. Can we first activate it and then
grab its pending? IOW, change cwq_activate_first_delayed() to
cwq_activate_delayed() which takes @work instead and then call it with
the work item being grabbed in try_to_grab_pending() before dec'ing
nr_in_flight?
Thanks.
--
tejun
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