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From: Keith Busch <[email protected]>
To: Jens Axboe <[email protected]>
Cc: [email protected], [email protected], [email protected]
Subject: Re: [PATCH 4/6] block: clear BIO_PERCPU_CACHE flag if polling isn't supported
Date: Fri, 13 Aug 2021 13:17:22 -0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On Thu, Aug 12, 2021 at 11:41:58AM -0600, Jens Axboe wrote:
> Indeed. Wonder if we should make that a small helper, as any clear of
> REQ_HIPRI should clear BIO_PERCPU_CACHE as well.
> 
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 7e852242f4cc..d2722ecd4d9b 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -821,11 +821,8 @@ static noinline_for_stack bool submit_bio_checks(struct bio *bio)
>  		}
>  	}
>  
> -	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags)) {
> -		/* can't support alloc cache if we turn off polling */
> -		bio_clear_flag(bio, BIO_PERCPU_CACHE);
> -		bio->bi_opf &= ~REQ_HIPRI;
> -	}
> +	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
> +		bio_clear_hipri(bio);

Since BIO_PERCPU_CACHE doesn't work without REQ_HIRPI, should this check
look more like this?

	if (!test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
		bio->bi_opf &= ~REQ_HIPRI;
	if (!(bio->bi_opf & REQ_HIPRI))
		bio_clear_flag(bio, BIO_PERCPU_CACHE);

I realise the only BIO_PERCPU_CACHE user in this series never sets it
without REQ_HIPRI, but it looks like a problem waiting to happen if
nothing enforces this pairing: someone could set the CACHE flag on a
QUEUE_FLAG_POLL enabled queue without setting HIPRI and get the wrong
bio_put() action.

  reply	other threads:[~2021-08-13 20:17 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12 15:41 [PATCHSET v5 0/6] Enable bio recycling for polled IO Jens Axboe
2021-08-12 15:41 ` [PATCH 1/6] bio: optimize initialization of a bio Jens Axboe
2021-08-12 16:16   ` Christoph Hellwig
2021-08-12 15:41 ` [PATCH 2/6] fs: add kiocb alloc cache flag Jens Axboe
2021-08-12 16:20   ` Christoph Hellwig
2021-08-12 15:41 ` [PATCH 3/6] bio: add allocation cache abstraction Jens Axboe
2021-08-12 16:32   ` Christoph Hellwig
2021-08-12 16:39     ` Jens Axboe
2021-08-12 15:41 ` [PATCH 4/6] block: clear BIO_PERCPU_CACHE flag if polling isn't supported Jens Axboe
2021-08-12 16:33   ` Christoph Hellwig
2021-08-12 17:31   ` Keith Busch
2021-08-12 17:41     ` Jens Axboe
2021-08-13 20:17       ` Keith Busch [this message]
2021-08-13 20:19         ` Jens Axboe
2021-08-13 20:31           ` Keith Busch
2021-08-13 20:33             ` Jens Axboe
2021-08-12 15:41 ` [PATCH 5/6] io_uring: enable use of bio alloc cache Jens Axboe
2021-08-12 16:39   ` Christoph Hellwig
2021-08-12 16:46     ` Jens Axboe
2021-08-12 15:41 ` [PATCH 6/6] block: use the percpu bio cache in __blkdev_direct_IO Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2021-08-11 19:35 [PATCHSET v4 0/6] Enable bio recycling for polled IO Jens Axboe
2021-08-11 19:35 ` [PATCH 4/6] block: clear BIO_PERCPU_CACHE flag if polling isn't supported Jens Axboe

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