linux-kernel - Re: [GIT PULL] bcachefs

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Date:   Fri, 11 Aug 2023 00:03:10 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org, dchinner@...hat.com,
        sandeen@...hat.com, willy@...radead.org, josef@...icpanda.com,
        tytso@....edu, bfoster@...hat.com, jack@...e.cz,
        andreas.gruenbacher@...il.com, brauner@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org,
        dhowells@...hat.com, snitzer@...nel.org, axboe@...nel.dk
Subject: Re: [GIT PULL] bcachefs

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.

The issue here is just that killing off a process should release all the
references it holds, and if we kill off all processes accessing a
filesystem we should be able to unmount it - but in this case we can't,
because fputs() are being delayed asynchronously.

delayed_fput() from AIO turned out to not be an issue in my testing, for
reasons that are unclear to me; flush_delayed_fput() certainly isn't
called in any relevant codepaths. The code _looks_ buggy to me, but I
wasn't able to trigger the bug with AIO.

io_uring adds its own layer of indirect asynchronous reference holding,
and that's why the issue crops up there - but io_uring isn't using
delayed_fput() either.

The patch I posted was to make sure the file ref doesn't outlive the
task - I honestly don't know what you and Jens don't like about that
approach (obviously, adding task->ref gets and puts to fastpaths is a
nonstarter, but that's fixable as mentioned).