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

On Sep 12, 2024, at 11:27, Muchun Song muchun.song@linux.dev wrote:

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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:

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On 9/3/24 2:16 AM, Muchun Song wrote:

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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.

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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?

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/**

• 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);

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Thanks, Ming