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

[Jens Axboe]

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.

Patch looks fine, though.

-- 
Jens Axboe