linux-ext4 - Re: [PATCH] ext4: fix warning when submitting superblock in ext4_commit

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Message-ID: <20220519093035.2kazqodrv4nqauwf@quack3.lan>
Date:   Thu, 19 May 2022 11:30:35 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <ritesh.list@...il.com>
Cc:     Zhang Yi <yi.zhang@...wei.com>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, yukuai3@...wei.com
Subject: Re: [PATCH] ext4: fix warning when submitting superblock in
 ext4_commit_super()

On Thu 19-05-22 11:59:29, Ritesh Harjani wrote:
> On 22/05/19 11:13AM, Zhang Yi wrote:
> > On 2022/5/19 1:06, Ritesh Harjani wrote:
> > > On 22/05/18 10:10PM, Zhang Yi wrote:
> > >> We have already check the io_error and uptodate flag before submitting
> > >> the superblock buffer, and re-set the uptodate flag if it has been
> > >> failed to write out. But it was lockless and could be raced by another
> > >> ext4_commit_super(), and finally trigger '!uptodate' WARNING when
> > >> marking buffer dirty. Fix it by submit buffer directly.
> > >
> > > I agree that there could be a race with multiple processes trying to call
> > > ext4_commit_super(). Do you have a easy reproducer for this issue?
> > >
> >
> > Sorry, I don't have a easy reproducer, but we can always reproduce it through
> > inject delay and add filters into the ext4_commit_super().

...
 
> > > Also do you think something like below should fix the problem too?
> > > So if you lock the buffer from checking until marking the buffer dirty, that
> > > should avoid the race too that you are reporting.
> > > Thoughts?
> > >
> >
> > Thanks for your suggestion. I've thought about this solution and yes it's simpler
> > to fix the race, but I think we lock and unlock the sbh several times just for
> > calling standard buffer write helpers is not so good. Opencode the submit
> > procedure looks more clear to me.
> 
> I agree your solution was cleaner since it does not has a lot of lock/unlock.
> My suggestion came in from looking at the history.
> This lock was added here [1] and I think it somehow got removed in this patch[2]
> 
> [1]: https://lore.kernel.org/linux-ext4/1467285150-15977-2-git-send-email-pranjas@gmail.com/
> [2]: https://lore.kernel.org/linux-ext4/20201216101844.22917-5-jack@suse.cz/

So the reason why I've move unlock_buffer() into ext4_update_super() was
mostly so that the function does not return with buffer lock (which is an
odd calling convention) when I was adding another user of it
(flush_stashed_error_work()).

> Rather then solutions, I had few queries :)
> 1. What are the implications of not using
> mark_buffer_dirty()/__sync_dirty_buffer()

Not much. Using submit_bh() directly is fine. Just the duplication of the
checks is somewhat unpleasant.

> 2. In your solution one thing which I was not clear of, was whether we
> should call clear_buffer_dirty() before calling submit_bh(), in case if
> somehow(?) the state of the buffer was already marked dirty? Not sure how
> this can happen, but I see the logic in mark_buffer_dirty() which checks,
> if the buffer is already marked dirty, it simply returns. Then
> __sync_dirty_buffer() clears the buffer dirty state.

It could happen e.g. if there was journalled update of the superblock
before. I guess calling clear_buffer_dirty() before submit_bh() does no
harm.

Otherwise I like Yi's solution.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR