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Date:   Mon, 21 Jun 2021 09:25:02 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Mike Snitzer <snitzer@redhat.com>,
        linux-block@vger.kernel.org,
        Jeffle Xu <jefflexu@linux.alibaba.com>, dm-devel@redhat.com,
        Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC PATCH V2 2/3] block: add ->poll_bio to
 block_device_operations
Message-ID: <20210621072502.GC6651@lst.de>
References: <20210617103549.930311-1-ming.lei@redhat.com> <20210617103549.930311-3-ming.lei@redhat.com>
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> +	struct gendisk *disk = bio->bi_bdev->bd_disk;
> +	struct request_queue *q = disk->queue;
>  	blk_qc_t cookie = READ_ONCE(bio->bi_cookie);
>  	int ret;
>  
> -	if (cookie == BLK_QC_T_NONE || !blk_queue_poll(q))
> +	if ((queue_is_mq(q) && cookie == BLK_QC_T_NONE) ||
> +			!blk_queue_poll(q))
>  		return 0;

How does polling for a bio without a cookie make sense even when
polling bio based?

But if we come up for a good rationale for this I'd really
split the conditions to make them more readable:

	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
		return 0;
	if (queue_is_mq(q) && cookie == BLK_QC_T_NONE)
		return 0;

> +	if (!queue_is_mq(q)) {
> +		if (disk->fops->poll_bio) {
> +			ret = disk->fops->poll_bio(bio, flags);
> +		} else {
> +			WARN_ON_ONCE(1);
> +			ret = 0;
> +		}
> +	} else {
>  		ret = blk_mq_poll(q, cookie, flags);

I'd go for someting like:

	if (queue_is_mq(q))
		ret = blk_mq_poll(q, cookie, flags);
	else if (disk->fops->poll_bio)
		ret = disk->fops->poll_bio(bio, flags);
	else
		WARN_ON_ONCE(1);

with ret initialized to 0 at declaration time.

>  struct block_device_operations {
>  	void (*submit_bio)(struct bio *bio);
> +	/* ->poll_bio is for bio driver only */

I'd drop the comment, this is already nicely documented in add_disk
together with the actual check.  We also don't note this for submit_bio
here.