linux-kernel - Re: [PATCH next] btrfs: fix deadlock in btrfs_read_chunk

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Message-ID: <20250625234420.1798-1-hdanton@sina.com>
Date: Thu, 26 Jun 2025 07:44:18 +0800
From: Hillf Danton <hdanton@...a.com>
To: Qu Wenruo <quwenruo.btrfs@....com>
Cc: Edward Adam Davis <eadavis@...com>,
	syzbot+fa90fcaa28f5cd4b1fc1@...kaller.appspotmail.com,
	clm@...com,
	josef@...icpanda.com,
	linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH next] btrfs: fix deadlock in btrfs_read_chunk_tree

On Thu, 26 Jun 2025 06:59:14 +0930 Qu Wenruo wrote:
> =E5=9C=A8 2025/6/25 22:10, Hillf Danton =E5=86=99=E9=81=93:
> > On Wed, 25 Jun 2025 20:19:06 +0930 Qu Wenruo wrote:
> >> =3DE5=3D9C=3DA8 2025/6/25 09:26, Hillf Danton =3DE5=3D86=3D99=3DE9=3D81=
> =3D93:
> >>> On Wed, 25 Jun 2025 06:00:09 +0930 Qu Wenruo wrote:
> >>>> =3D3DE5=3D3D9C=3D3DA8 2025/6/25 00:00, Edward Adam Davis =3D3DE5=3D3D=
> 86=3D3D99=3D3DE9=3D
> >> =3D3D81=3D3D93:
> >>>>> Remove the lock uuid_mutex outside of sget_fc() to avoid the deadloc=
> k
> >>>>> reported by [1].
> >>>>> =3D3D20
> >>>>> [1]
> >>>>> -> #1 (&type->s_umount_key#41/1){+.+.}-{4:4}:
> >>>>>           lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
> >>>>>           down_write_nested+0x9d/0x200 kernel/locking/rwsem.c:1693
> >>>>>           alloc_super+0x204/0x970 fs/super.c:345
> >>> =3D20
> >>> Given kzalloc [3], the syzbot report is false positive (a known lockde=
> p
> >>> issue) as nobody else should acquire s->s_umount lock.
> >>> =3D20
> >>> [3] https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-nex=
> t.=3D
> >> git/tree/fs/super.c?id=3D3D7aacdf6feed1#n319
> >>
> >> Not a false alert either.
> >>
> >> sget_fc() can return an existing super block, we can race between a=3D2=
> 0
> >> mount and an unmount on the same super block.
> >>
> >> In that case it's going to cause problem.
> >>
> >> This is already fixed in the v4 (and later v5) patchset:
> >>
> >> https://lore.kernel.org/linux-btrfs/cover.1750724841.git.wqu@suse.com/
> >>
> > Can v5 survive the syzbot test?
> 
> Yes, I enabled lockdep during v5 tests.
> 
Fine, feel free to show us the Tested-by syzbot gave you.