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

Muchun Song
2024-09-12T06:27:56Z

> 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