linux-kernel - Re: [PATCH] bcache: fix double bio_endio completion in detached_dev_end

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Message-ID: <CANubcdUdQ9gJ7uQELc80h0+FpurR5f2COmB3hBEDejavfFZJ9g@mail.gmail.com>
Date: Thu, 15 Jan 2026 16:06:53 +0800
From: Stephen Zhang <starzhangzsd@...il.com>
To: colyli@...as.com, kent.overstreet@...ux.dev, axboe@...nel.dk, 
	sashal@...nel.org
Cc: linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org, 
	zhangshida@...inos.cn
Subject: Re: [PATCH] bcache: fix double bio_endio completion in detached_dev_end_io

zhangshida <starzhangzsd@...il.com> 于2026年1月15日周四 15:48写道：
>
> From: Shida Zhang <zhangshida@...inos.cn>
>
> Commit 53280e398471 ("bcache: fix improper use of bi_end_io") attempted
> to fix up bio completions by replacing manual bi_end_io calls with
> bio_endio(). However, it introduced a double-completion bug in the
> detached_dev path.
>
> In a normal completion path, the call stack is:
>    blk_update_request
>      bio_endio(bio)
>        bio->bi_end_io(bio) -> detached_dev_end_io
>          bio_endio(bio)    <- BUG: second call
>
> To fix this, detached_dev_end_io() must manually call the next completion
> handler in the chain.
>
> However, in detached_dev_do_request(), if a discard is unsupported, the
> bio is rejected before being submitted to the lower level. In this case,
> we can use the standard bio_endio().
>
>    detached_dev_do_request
>      bio_endio(bio)        <- Correct: starts completion for
>                                 unsubmitted bio
>
> Fixes: 53280e398471 ("bcache: fix improper use of bi_end_io")
> Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> ---
>  drivers/md/bcache/request.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index 82fdea7dea7..ec712b5879f 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -1104,7 +1104,14 @@ static void detached_dev_end_io(struct bio *bio)
>         }
>
>         kfree(ddip);
> -       bio_endio(bio);
> +       /*
> +        * This is an exception where bio_endio() cannot be used.
> +        * We are already called from within a bio_endio() stack;
> +        * calling it again here would result in a double-completion
> +        * (decrementing bi_remaining twice). We must call the
> +        * original completion routine directly.
> +        */
> +       bio->bi_end_io(bio);
>  }
>
>  static void detached_dev_do_request(struct bcache_device *d, struct bio *bio,
> @@ -1136,7 +1143,7 @@ static void detached_dev_do_request(struct bcache_device *d, struct bio *bio,
>
>         if ((bio_op(bio) == REQ_OP_DISCARD) &&
>             !bdev_max_discard_sectors(dc->bdev))
> -               detached_dev_end_io(bio);
> +               bio_endio(bio);
>         else
>                 submit_bio_noacct(bio);
>  }
> --
> 2.34.1
>

Hi,

My apologies for the late reply due to a delay in checking my working inbox.
I see the issue mentioned in:
https://lore.kernel.org/all/aWU2mO5v6RezmIpZ@moria.home.lan/
this was indeed an oversight on my part.

To resolve this quickly, I've prepared a direct fix for the
double-completion bug.
I hope this is better than a full revert.

Thank,
Shida