Re: [PATCH v2 2/3] block: fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests

[Ming Lei <ming.lei@redhat.com>]

On Tue, Sep 10, 2024 at 07:22:16AM -0600, Jens Axboe wrote:
> On 9/3/24 2:16 AM, Muchun Song wrote:
> > Supposing the following scenario.
> > 
> > CPU0                                        CPU1
> > 
> > blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
> > blk_mq_run_hw_queue()                       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
> >     if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
> >         return                                      blk_mq_run_hw_queue()
> >     blk_mq_sched_dispatch_requests()                    if (!blk_mq_hctx_has_pending())     4) load
> >                                                            return
> > 
> > The full memory barrier should be inserted between 1) and 2), as well as
> > between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
> > cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
> > Otherwise, either CPU will not re-run the hardware queue causing starvation.
> > 
> > So the first solution is to 1) add a pair of memory barrier to fix the
> > problem, another solution is to 2) use hctx->queue->queue_lock to synchronize
> > QUEUE_FLAG_QUIESCED. Here, we chose 2) to fix it since memory barrier is not
> > easy to be maintained.
> 
> Same comment here, 72-74 chars wide please.
> 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index b2d0f22de0c7f..ac39f2a346a52 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2202,6 +2202,24 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
> >  }
> >  EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
> >  
> > +static inline bool blk_mq_hw_queue_need_run(struct blk_mq_hw_ctx *hctx)
> > +{
> > +	bool need_run;
> > +
> > +	/*
> > +	 * When queue is quiesced, we may be switching io scheduler, or
> > +	 * updating nr_hw_queues, or other things, and we can't run queue
> > +	 * any more, even blk_mq_hctx_has_pending() can't be called safely.
> > +	 *
> > +	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
> > +	 * quiesced.
> > +	 */
> > +	__blk_mq_run_dispatch_ops(hctx->queue, false,
> > +				  need_run = !blk_queue_quiesced(hctx->queue) &&
> > +					      blk_mq_hctx_has_pending(hctx));
> > +	return need_run;
> > +}
> 
> This __blk_mq_run_dispatch_ops() is also way too wide, why didn't you
> just break it like where you copied it from?
> 
> > +
> >  /**
> >   * blk_mq_run_hw_queue - Start to run a hardware queue.
> >   * @hctx: Pointer to the hardware queue to run.
> > @@ -2222,20 +2240,23 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> >  
> >  	might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);
> >  
> > -	/*
> > -	 * When queue is quiesced, we may be switching io scheduler, or
> > -	 * updating nr_hw_queues, or other things, and we can't run queue
> > -	 * any more, even __blk_mq_hctx_has_pending() can't be called safely.
> > -	 *
> > -	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
> > -	 * quiesced.
> > -	 */
> > -	__blk_mq_run_dispatch_ops(hctx->queue, false,
> > -		need_run = !blk_queue_quiesced(hctx->queue) &&
> > -		blk_mq_hctx_has_pending(hctx));
> > +	need_run = blk_mq_hw_queue_need_run(hctx);
> > +	if (!need_run) {
> > +		unsigned long flags;
> >  
> > -	if (!need_run)
> > -		return;
> > +		/*
> > +		 * synchronize with blk_mq_unquiesce_queue(), becuase we check
> > +		 * if hw queue is quiesced locklessly above, we need the use
> > +		 * ->queue_lock to make sure we see the up-to-date status to
> > +		 * not miss rerunning the hw queue.
> > +		 */
> > +		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
> > +		need_run = blk_mq_hw_queue_need_run(hctx);
> > +		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
> > +
> > +		if (!need_run)
> > +			return;
> > +	}
> 
> Is this not solvable on the unquiesce side instead? It's rather a shame
> to add overhead to the fast path to avoid a race with something that's
> super unlikely, like quisce.

Yeah, it can be solved by adding synchronize_rcu()/srcu() in unquiesce
side, but SCSI may call it in non-sleepable context via scsi_internal_device_unblock_nowait().


Thanks,
Ming