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

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Message-ID: <aWiqMzQ9PIWFfyfP@moria.home.lan>
Date: Thu, 15 Jan 2026 03:59:00 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Stephen Zhang <starzhangzsd@...il.com>
Cc: colyli@...as.com, axboe@...nel.dk, sashal@...nel.org, 
	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

On Thu, Jan 15, 2026 at 04:06:53PM +0800, Stephen Zhang wrote:
> 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.

In general, it's just safer, simpler and saner to revert, reverting a
patch is not something to be avoided. If there's _any_ new trickyness
required in the fix, it's better to just revert than rush things.

I revert or kick patches out - including my own - all the time.

That said, this patch is good, you've got a comment explaining what's
going on. Christoph's version of just always cloning the bio is
definitely cleaner, but that's a bigger change,