linux-ext4 - Re: possible deadlock in __generic_file

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Date:   Sat, 23 Mar 2019 08:16:36 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+5cd33f0e6abe2bb3e397@...kaller.appspotmail.com>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: possible deadlock in __generic_file_fsync

On Fri, Mar 22, 2019 at 10:28 PM syzbot
<syzbot+5cd33f0e6abe2bb3e397@...kaller.appspotmail.com> wrote:
>
> syzbot has bisected this bug to:
>
> commit 9022ada8ab6f1f1a932a3c93815061042e6548a5
> Author: Linus Torvalds <torvalds@...ux-foundation.org>
> Date:   Fri Aug 24 20:16:36 2018 +0000
>
>      Merge branch 'for-4.19' of
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15330437200000
> start commit:   270b77a0 Merge tag 'drm-fixes-2018-10-20-1' of git://anong..
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=17330437200000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13330437200000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b3f55cb3dfcc6c33
> dashboard link: https://syzkaller.appspot.com/bug?extid=5cd33f0e6abe2bb3e397
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1436fc45400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11058e2d400000
>
> Reported-by: syzbot+5cd33f0e6abe2bb3e397@...kaller.appspotmail.com
> Fixes: 9022ada8ab6f ("Merge branch 'for-4.19' of
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq")
>
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection



This is a lockdep-detected bug, but it is reproduced with very low probability:

testing commit 270b77a0f30e7bc61a9081b86d74dbb62fa6a69d with gcc (GCC) 8.1.0
run #0: crashed: possible deadlock in __generic_file_fsync
run #1: OK
run #2: OK
run #3: OK
run #4: OK
run #5: OK
run #6: OK
run #7: OK
run #8: OK
run #9: OK

I would expect that for lockdep it's only enough to trigger each path
involved in the deadlock once. Why is it so hard to reproduce then? Is
it something to improve in lockdep?