Re: [PATCH] bcache: fix double bio_endio completion in detached_dev_end_io

Stephen Zhang
2026-01-15T08:07:30Z

zhangshida <starzhangzsd@gmail.com> 于2026年1月15日周四 15:48写道：
>
> From: Shida Zhang <zhangshida@kylinos.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@kylinos.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