Re: [PATCH v4 2/2] block: cancel all throttled bios in del_gendisk()

[yukuai (C)]

在 2021/12/02 22:48, Michal Koutný 写道:
> Hello Kuai.
>
> On Thu, Dec 02, 2021 at 09:04:40PM +0800, Yu Kuai <yukuai3@xxxxxxxxxx> wrote:
> > For example, if user thread is throttled with low bps while it's
> > issuing large io, and the device is deleted. The user thread will
> > wait for a long time for io to return.
>
> Do I understand correctly the "long time" here is
> outstanding_IO_size/throttled_bandwidth? Or are you getting at some

Hi, Michal

Yes, this is exactly what I mean.
> other cause/longer time?
>
> > +void blk_throtl_cancel_bios(struct request_queue *q)
> > +{
> > +	struct throtl_data *td = q->td;
> > +	struct bio_list bio_list_on_stack;
> > +	struct blkcg_gq *blkg;
> > +	struct cgroup_subsys_state *pos_css;
> > +	struct bio *bio;
> > +	int rw;
> > +
> > +	bio_list_init(&bio_list_on_stack);
> > +
> > +	/*
> > +	 * hold queue_lock to prevent concurrent with dispatching
> > +	 * throttled bios by timer.
> > +	 */
> > +	spin_lock_irq(&q->queue_lock);
>
> You've replaced the rcu_read_lock() with the queue lock but...
>
> > +
> > +	/*
> > +	 * Drain each tg while doing post-order walk on the blkg tree, so
> > +	 * that all bios are propagated to td->service_queue.  It'd be
> > +	 * better to walk service_queue tree directly but blkg walk is
> > +	 * easier.
> > +	 */
> > +	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
> > +		tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
>
> ...you also need the rcu_read_lock() here since you may encounter a
> (descendant) blkcg that's removed concurrently.

blkg_destroy() is protected by the queue_lock，so I think queue_lock can
protect such concurrent scenario.

Thanks,
Kuai
> (I may miss some consequences of doing this under the queue_lock so if
> the concurrent removal is ruled out, please make a comment about it.)
>
>
> Regards,
> Michal