On Tue, Oct 12, 2021 at 01:12:24PM +0200, Christoph Hellwig wrote:
> Replace the blk_poll interface that requires the caller to keep a queue
> and cookie from the submissions with polling based on the bio.
>
> Polling for the bio itself leads to a few advantages:
>
> - the cookie construction can made entirely private in blk-mq.c
> - the caller does not need to remember the request_queue and cookie
> separately and thus sidesteps their lifetime issues
> - keeping the device and the cookie inside the bio allows to trivially
> support polling BIOs remapping by stacking drivers
> - a lot of code to propagate the cookie back up the submission path can
> be removed entirely.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Tested-by: Mark Wunderlich <mark.wunderlich@xxxxxxxxx>
> ---
...
> +/**
> + * bio_poll - poll for BIO completions
> + * @bio: bio to poll for
> + * @flags: BLK_POLL_* flags that control the behavior
> + *
> + * Poll for completions on queue associated with the bio. Returns number of
> + * completed entries found.
> + *
> + * Note: the caller must either be the context that submitted @bio, or
> + * be in a RCU critical section to prevent freeing of @bio.
> + */
> +int bio_poll(struct bio *bio, unsigned int flags)
> +{
> + struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> + blk_qc_t cookie = READ_ONCE(bio->bi_cookie);
> + int ret;
> +
> + if (cookie == BLK_QC_T_NONE ||
> + !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> + return 0;
> +
> + if (current->plug)
> + blk_flush_plug_list(current->plug, false);
> +
> + if (blk_queue_enter(q, BLK_MQ_REQ_NOWAIT))
> + return 0;
Now bdev, gendisk and request_queue are freed after RCU grace period, so this
way is safe since 340e84573878 ("block: delay freeing the gendisk").
Thanks,
Ming
