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

[Muchun Song]

> On Sep 10, 2024, at 21:22, Jens Axboe <axboe@xxxxxxxxx> wrote:
> 
> On 9/3/24 2:16 AM, Muchun Song wrote:
>> Supposing first scenario with a virtio_blk driver.
>> 
>> CPU0                                                                CPU1
>> 
>> blk_mq_try_issue_directly()
>>    __blk_mq_issue_directly()
>>        q->mq_ops->queue_rq()
>>            virtio_queue_rq()
>>                blk_mq_stop_hw_queue()
>>                                                                    virtblk_done()
>>    blk_mq_request_bypass_insert()                                      blk_mq_start_stopped_hw_queues()
>>        /* Add IO request to dispatch list */   1) store                    blk_mq_start_stopped_hw_queue()
>>                                                                                clear_bit(BLK_MQ_S_STOPPED)                 3) store
>>    blk_mq_run_hw_queue()                                                       blk_mq_run_hw_queue()
>>        if (!blk_mq_hctx_has_pending())                                             if (!blk_mq_hctx_has_pending())         4) load
>>            return                                                                      return
>>        blk_mq_sched_dispatch_requests()                                            blk_mq_sched_dispatch_requests()
>>            if (blk_mq_hctx_stopped())          2) load                                 if (blk_mq_hctx_stopped())
>>                return                                                                     return
>>            __blk_mq_sched_dispatch_requests()                                          __blk_mq_sched_dispatch_requests()
>> 
>> Supposing another scenario.
>> 
>> CPU0                                                                CPU1
>> 
>> blk_mq_requeue_work()
>>    /* Add IO request to dispatch list */       1) store            virtblk_done()
>>    blk_mq_run_hw_queues()/blk_mq_delay_run_hw_queues()                 blk_mq_start_stopped_hw_queues()
>>        if (blk_mq_hctx_stopped())              2) load                     blk_mq_start_stopped_hw_queue()
>>            continue                                                            clear_bit(BLK_MQ_S_STOPPED)                 3) store
>>        blk_mq_run_hw_queue()/blk_mq_delay_run_hw_queue()                       blk_mq_run_hw_queue()
>>                                                                                    if (!blk_mq_hctx_has_pending())         4) load
>>                                                                                        return
>>                                                                                    blk_mq_sched_dispatch_requests()
>> 
>> Both scenarios are similar, 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
>> BLK_MQ_S_STOPPED is cleared or CPU1 sees dispatch list. Otherwise, either CPU
>> will not rerun the hardware queue causing starvation of the request.
>> 
>> The easy way to fix it is to add the essential full memory barrier into helper
>> of blk_mq_hctx_stopped(). In order to not affect the fast path (hardware queue
>> is not stopped most of the time), we only insert the barrier into the slow path.
>> Actually, only slow path needs to care about missing of dispatching the request
>> to the low-level device driver.
> 
> Again, this is way too wide, it's unreadable.

OK. Will do.

Muchun,
Thanks.

> 
> Patch looks fine, though.
> 
> -- 
> Jens Axboe