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

Jens Axboe
2024-09-10T13:22:18Z

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.

--
Jens Axboe