Re: [PATCH next] btrfs: fix deadlock in btrfs_read_chunk_tree

[Hillf Danton]

On Wed, 25 Jun 2025 20:19:06 +0930 Qu Wenruo wrote:
> =E5=9C=A8 2025/6/25 09:26, Hillf Danton =E5=86=99=E9=81=93:
> > On Wed, 25 Jun 2025 06:00:09 +0930 Qu Wenruo wrote:
> >> =3DE5=3D9C=3DA8 2025/6/25 00:00, Edward Adam Davis =3DE5=3D86=3D99=3DE9=
> =3D81=3D93:
> >>> Remove the lock uuid_mutex outside of sget_fc() to avoid the deadlock
> >>> reported by [1].
> >>> =3D20
> >>> [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
> >=20
> > Given kzalloc [3], the syzbot report is false positive (a known lockdep
> > issue) as nobody else should acquire s->s_umount lock.
> >=20
> > [3] https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.=
> git/tree/fs/super.c?id=3D7aacdf6feed1#n319
> 
> Not a false alert either.
> 
> sget_fc() can return an existing super block, we can race between a=20
> 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@xxxxxxxx/
> 
Can v5 survive the syzbot test?