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

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

> On Sep 10, 2024, at 21:22, Jens Axboe <axboe@kernel.dk> 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.

OK.

> 
>> 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?

I thought the rule allows max 80 chars pre line, so I adjusted
the code to let them align with the above "(". Seems you prefer
the previous way, I can keep it the same as before.

Muchun,
Thanks.

> 
>> +
>> /**
>>  * 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.
> 
> -- 
> Jens Axboe