Re: [PATCH] block: don't protect submit_bio_checks by q_usage_counter

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jan 04, 2022 at 09:42:23PM +0800, Ming Lei wrote:
> Commit cc9c884dd7f4 ("block: call submit_bio_checks under q_usage_counter")
> uses q_usage_counter to protect submit_bio_checks for avoiding IO after
> disk is deleted by del_gendisk().
> 
> Turns out the protection isn't necessary, because once
> blk_mq_freeze_queue_wait() in del_gendisk() returns:
> 
> 1) all in-flight IO has been done
> 
> 2) all new IO will be failed in __bio_queue_enter() because
>    q_usage_counter is dead, and GD_DEAD is set
> 
> 3) both disk and request queue instance are safe since caller of
> submit_bio() guarantees that the disk can't be closed.
> 
> Once submit_bio_checks() needn't the protection of q_usage_counter, we can
> move submit_bio_checks before calling blk_mq_submit_bio() and
> ->submit_bio(). With this change, we needn't to throttle queue with
> holding one allocated request, then precise driver tag or request won't be
> wasted in throttling. Meantime we can unify the bio check for both bio
> based and request based driver.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

Hello,

Any comments on this fix?

-- 
Ming