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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Aug 2023 22:20:22 -0700
Message-ID: <CAHk-=whDuBPONoTMRQn2aX64uYTG5E3QaZ4abJStYRHFMMToyw@mail.gmail.com>
Subject: Re: [GIT PULL] bcachefs
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     "Darrick J. Wong" <djwong@kernel.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-bcachefs@vger.kernel.org, dchinner@redhat.com,
        sandeen@redhat.com, willy@infradead.org, josef@toxicpanda.com,
        tytso@mit.edu, bfoster@redhat.com, jack@suse.cz,
        andreas.gruenbacher@gmail.com, brauner@kernel.org,
        peterz@infradead.org, akpm@linux-foundation.org,
        dhowells@redhat.com, snitzer@kernel.org, axboe@kernel.dk
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On Thu, 10 Aug 2023 at 21:03, Kent Overstreet <kent.overstreet@linux.dev> wrote:
>
> On Thu, Aug 10, 2023 at 04:47:22PM -0700, Linus Torvalds wrote:
> > So I might be barking up entirely the wrong tree.
>
> Yeah, I think you are, it sounds like you're describing an entirely
> different sort of race.

I was just going by Darrick's description of what he saw, which
*seemed* to be that umount had finished with stuff still active:

  "Here, umount exits before the filesystem is really torn down, and then
  mount fails because it can't get an exclusive lock on the device."

But maybe I misunderstood, and the umount wasn't actually successful
(ie "exits" may have been "failed with EBUSY")?

So I was trying to figure out what could cause the behavior I thought
Darrick was describing, which would imply a mnt_count issue.

If it's purely "umount doesnt' succeed because the filesystem is still
busy with cleanups", then things are much better.

The mnt_count is nasty, if it's not that, we're actually much better
off, and I'll be very happy to have misunderstood Darrick's case.

              Linus