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

[Muchun Song <muchun.song@linux.dev>]

> On Sep 12, 2024, at 11:27, Muchun Song <muchun.song@linux.dev> wrote:
> 
> 
> 
>> On Sep 11, 2024, at 11:54, Ming Lei <ming.lei@redhat.com> wrote:
>> 
>> 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().
> 
> Hi Ming and Jens,
> 
> I use call_srcu/call_rcu to make it non-sleepable. Does this make sense to you?

Sorry for the noise. call_srcu/call_rcu can't be easy to do this.
Because call_srcu/call_rcu could be issued twice if users try to
unquiesce the queue again before the callback of
blk_mq_run_hw_queues_rcu has been executed.

Thanks.

> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 12bf38bec1044..86cdff28b2ce6 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -247,6 +247,13 @@ void blk_mq_quiesce_queue(struct request_queue *q)
> }
> EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
> 
> +static void blk_mq_run_hw_queues_rcu(struct rcu_head *rh)
> +{
> +       struct request_queue *q = container_of(rh, struct request_queue,
> +                                              rcu_head);
> +       blk_mq_run_hw_queues(q, true);
> +}
> +
> /*
>  * blk_mq_unquiesce_queue() - counterpart of blk_mq_quiesce_queue()
>  * @q: request queue.
> @@ -269,8 +276,13 @@ void blk_mq_unquiesce_queue(struct request_queue *q)
>        spin_unlock_irqrestore(&q->queue_lock, flags);
> 
>        /* dispatch requests which are inserted during quiescing */
> -       if (run_queue)
> -               blk_mq_run_hw_queues(q, true);
> +       if (run_queue) {
> +               if (q->tag_set->flags & BLK_MQ_F_BLOCKING)
> +                       call_srcu(q->tag_set->srcu, &q->rcu_head,
> +                                 blk_mq_run_hw_queues_rcu);
> +               else
> +                       call_rcu(&q->rcu_head, blk_mq_run_hw_queues_rcu);
> +       }
> }
> EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
> 
>> 
>> 
>> Thanks,
>> Ming