Re: [PATCH 2/4] block: split bio_queue_enter from blk_queue_enter

[Bart Van Assche <bvanassche@acm.org>]

On 9/20/21 04:24, Christoph Hellwig wrote:
> To prepare for fixing a gendisk shutdown race, open code the
> blk_queue_enter logic in bio_queue_enter.  This also remove the
                                                        ^^^^^^
                                                        removes
> pointless flags translation.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   block/blk-core.c | 28 +++++++++++++++++++++-------
>   1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 4c078681f39b8..be7cd1819b605 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -475,18 +475,32 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
>   static inline int bio_queue_enter(struct bio *bio)
>   {
>   	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> -	bool nowait = bio->bi_opf & REQ_NOWAIT;
> -	int ret;
>   
> -	ret = blk_queue_enter(q, nowait ? BLK_MQ_REQ_NOWAIT : 0);
> -	if (unlikely(ret)) {
> -		if (nowait && !blk_queue_dying(q))
> +	while (!blk_try_enter_queue(q, false)) {
> +		if (bio->bi_opf & REQ_NOWAIT) {
>   			bio_wouldblock_error(bio);
> -		else
> +			return -EBUSY;
> +		}
> +
> +		/*
> +		 * read pair of barrier in blk_freeze_queue_start(), we need to
> +		 * order reading __PERCPU_REF_DEAD flag of .q_usage_counter and
> +		 * reading .mq_freeze_depth or queue dying flag, otherwise the
> +		 * following wait may never return if the two reads are
> +		 * reordered.
> +		 */
> +		smp_rmb();
> +		wait_event(q->mq_freeze_wq,
> +			   (!q->mq_freeze_depth &&
> +			    blk_pm_resume_queue(false, q)) ||
> +			   blk_queue_dying(q));
> +		if (blk_queue_dying(q)) {
>   			bio_io_error(bio);
> +			return -ENODEV;
> +		}
>   	}
>   
> -	return ret;
> +	return 0;

This patch changes the behavior of bio_queue_enter(). I think that
preserving the existing behavior implies testing blk_queue_dying(q)
before checking the REQ_NOWAIT flag.

Thanks,

Bart.