Re: [PATCH v2 4/8] blk-mq: fix blk_mq_quiesce_queue

Ming Lei
2017-05-30T00:22:49Z

On Sun, May 28, 2017 at 04:10:09PM +0000, Bart Van Assche wrote:
> On Sun, 2017-05-28 at 18:44 +0800, Ming Lei wrote:
> > On Sat, May 27, 2017 at 09:46:45PM +0000, Bart Van Assche wrote:
> > > On Sat, 2017-05-27 at 22:21 +0800, Ming Lei wrote:
> > > > bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)
> > > > @@ -1108,13 +1119,15 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> > > >
> > > > if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> > > > rcu_read_lock();
> > > > - blk_mq_sched_dispatch_requests(hctx);
> > > > + if (!blk_queue_quiesced(hctx->queue))
> > > > + blk_mq_sched_dispatch_requests(hctx);
> > > > rcu_read_unlock();
> > > > } else {
> > > > might_sleep();
> > > >
> > > > srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
> > > > - blk_mq_sched_dispatch_requests(hctx);
> > > > + if (!blk_queue_quiesced(hctx->queue))
> > > > + blk_mq_sched_dispatch_requests(hctx);
> > > > srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
> > > > }
> > > > }
> > >
> > > Sorry but I don't like these changes. Why have the blk_queue_quiesced()
> > > calls be added at other code locations than the blk_mq_hctx_stopped() calls?
> > > This will make the block layer unnecessary hard to maintain. Please consider
> > > to change the blk_mq_hctx_stopped(hctx) calls in blk_mq_sched_dispatch_requests()
> > > and *blk_mq_*run_hw_queue*() into blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q).
> >
> > One benefit is that we make it explicit that the flag has to be checked
> > inside the RCU read-side critical sections. If you put it somewhere,
> > someone may put it out of read-side critical sections in future.
>
> Hello Ming,
>
> I really would like to see the blk_queue_quiesced() tests as close as possible to
> the blk_mq_hctx_stopped() tests. But I agree that we need a way to document and/or

Could you explain why we have to do that? And checking on stopped state
doesn't need to hold RCU/SRCU read lock, and that two states are really
different.

> verify that these tests occur with an RCU read-side lock held. Have you considered
> to use rcu_read_lock_held() to document this?

Then we need to check if it is RCU or SRCU, and make code ugly as
current check on BLOCKING.

Thanks,
Ming