Re: [PATCH] blk-mq: Rework blk_mq_mark_tag_wait()

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On Wed, Jan 10, 2018 at 10:04:28AM -0800, Bart Van Assche wrote:
> Use prepare_to_wait() and finish_wait() instead of open-coding these
> functions. Reduce the number of if-statements to make
> blk_mq_mark_tag_wait() easier to read. Both add_wait_queue() and
> blk_mq_dispatch_wake() protect wait queue manipulations with the wait
> queue lock. Hence also protect the !list_empty(&wait->entry) test with
> the wait queue lock instead of the hctx lock.
> 
> Note: a side effect of this patch is that the task state is changed
> for shared queues before and after the blk_mq_get_driver_tag().
> Since blk_mq_dispatch_wake() ignores the task state these task state
> changes do not affect which task gets woken up.

I'm not a fan of this. The reason it's open-coded is exactly because we
don't want to mess around with the task state.

> See also commit f906a6a0f426 ("blk-mq: improve tag waiting setup for
> non-shared tags").
> 
> Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxx>
> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx>
> ---
>  block/blk-mq.c | 49 +++++++++++++++++--------------------------------
>  1 file changed, 17 insertions(+), 32 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 61bdbf45c3be..29f140b4dbf7 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1121,50 +1121,35 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
>  	if (!shared_tags) {
>  		if (!test_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state))
>  			set_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state);
> +		ret = blk_mq_get_driver_tag(rq, hctx, false);
> +		/*
> +		 * Don't clear RESTART here, someone else could have set it.
> +		 * At most this will cost an extra queue run.
> +		 */
>  	} else {
>  		wait = &this_hctx->dispatch_wait;
>  		if (!list_empty_careful(&wait->entry))
>  			return false;
>  
> -		spin_lock(&this_hctx->lock);
> -		if (!list_empty(&wait->entry)) {
> -			spin_unlock(&this_hctx->lock);
> -			return false;
> -		}
> -
>  		ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
> -		add_wait_queue(&ws->wait, wait);
> -	}
> -
> -	/*
> -	 * It's possible that a tag was freed in the window between the
> -	 * allocation failure and adding the hardware queue to the wait
> -	 * queue.
> -	 */
> -	ret = blk_mq_get_driver_tag(rq, hctx, false);
> +		prepare_to_wait(&ws->wait, wait, TASK_UNINTERRUPTIBLE);
>  
> -	if (!shared_tags) {
>  		/*
> -		 * Don't clear RESTART here, someone else could have set it.
> -		 * At most this will cost an extra queue run.
> +		 * It's possible that a tag was freed in the window between the
> +		 * allocation failure and adding the hardware queue to the wait
> +		 * queue.
>  		 */
> -		return ret;
> -	} else {
> -		if (!ret) {
> -			spin_unlock(&this_hctx->lock);
> -			return false;
> -		}
> -
> +		ret = blk_mq_get_driver_tag(rq, hctx, false);
>  		/*
> -		 * We got a tag, remove ourselves from the wait queue to ensure
> -		 * someone else gets the wakeup.
> +		 * If we got a tag, remove ourselves from the wait queue to
> +		 * ensure someone else gets the wakeup.
>  		 */
> -		spin_lock_irq(&ws->wait.lock);
> -		list_del_init(&wait->entry);
> -		spin_unlock_irq(&ws->wait.lock);
> -		spin_unlock(&this_hctx->lock);
> -		return true;
> +		if (ret)
> +			finish_wait(&ws->wait, wait);
> +		else
> +			__set_current_state(TASK_RUNNING);
>  	}
> +	return ret;
>  }
>  
>  bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> -- 
> 2.15.1
> 



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