Re: [PATCH V2] block: avoid io timeout in case of sync polled dio

[Christoph Hellwig <hch@lst.de>]

On Fri, Apr 15, 2022 at 11:47:03AM +0800, Ming Lei wrote:
> +	/* make sure the bio is issued before polling */
> +	if (bio.bi_opf & REQ_POLLED)
> +		blk_flush_plug(current->plug, false);

I still think the core code should handle this.  Without that we'd need
to export the blk_flush_plug for anything that would want to poll bios
from modules, in addition to it generally being a mess.  See a proposed
patch for that below.  I'd also split the flush aspect from the poll
aspect into two patches.

> +		if (bio.bi_opf & REQ_POLLED)
> +			bio_poll(&bio, NULL, 0);
> +		else
>  			blk_io_schedule();

Instead of this duplicate logic everywhere I'd just make bio_boll
call blk_io_schedule for the !REQ_POLLED case and simplify all the
callers.

> +			if (dio->submit.poll_bio &&
> +					(dio->submit.poll_bio->bi_opf &
> +						REQ_POLLED))

This indentation looks awfull,normal would be:

			if (dio->submit.poll_bio &&
			    (dio->submit.poll_bio->bi_opf & REQ_POLLED))

---
From 08ff61b0142eb708fc384cf867c72175561d974a Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Fri, 15 Apr 2022 07:15:42 +0200
Subject: blk-mq: don't plug for synchronously polled requests

For synchronous polling to work, the bio must be issued to the driver from
the submit_bio call, otherwise ->bi_cookie won't be set.

Based on a patch from Ming Lei.

Reported-by: Changhui Zhong <czhong@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-mq.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index ed3ed86f7dd24..bcc7e3d11296c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2851,7 +2851,13 @@ void blk_mq_submit_bio(struct bio *bio)
 		return;
 	}
 
-	if (plug)
+	/*
+	 * We can't plug for synchronously polled submissions, otherwise
+	 * bio->bi_cookie won't be set directly after submission, which is the
+	 * indicator used by the submitter to check if a bio needs polling.
+	 */
+	if (plug &&
+	    (rq->bio->bi_opf & (REQ_POLLED | REQ_NOWAIT)) != REQ_POLLED)
 		blk_add_rq_to_plug(plug, rq);
 	else if ((rq->rq_flags & RQF_ELV) ||
 		 (rq->mq_hctx->dispatch_busy &&
-- 
2.30.2