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

[Hillf Danton <hdanton@sina.com>]

On Wed, 25 Jun 2025 06:00:09 +0930 Qu Wenruo wrote:
> =E5=9C=A8 2025/6/25 00:00, Edward Adam Davis =E5=86=99=E9=81=93:
> > Remove the lock uuid_mutex outside of sget_fc() to avoid the deadlock
> > reported by [1].
> >=20
> > [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

Given kzalloc [3], the syzbot report is false positive (a known lockdep
issue) as nobody else should acquire s->s_umount lock.

[3] https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/fs/super.c?id=7aacdf6feed1#n319

> >         sget_fc+0x329/0xa40 fs/super.c:761
> >         btrfs_get_tree_super fs/btrfs/super.c:1867 [inline]
> >         btrfs_get_tree_subvol fs/btrfs/super.c:2059 [inline]
> >         btrfs_get_tree+0x4c6/0x12d0 fs/btrfs/super.c:2093
> >         vfs_get_tree+0x8f/0x2b0 fs/super.c:1804
> >         do_new_mount+0x24a/0xa40 fs/namespace.c:3902
> >         do_mount fs/namespace.c:4239 [inline]
> >         __do_sys_mount fs/namespace.c:4450 [inline]
> >         __se_sys_mount+0x317/0x410 fs/namespace.c:4427
> >         do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >         do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >         entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >=20
> > -> #0 (uuid_mutex){+.+.}-{4:4}:
> >         check_prev_add kernel/locking/lockdep.c:3168 [inline]
> >         check_prevs_add kernel/locking/lockdep.c:3287 [inline]
> >         validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3911
> >         __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5240
> >         lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
> >         __mutex_lock_common kernel/locking/mutex.c:602 [inline]
> >         __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
> >         btrfs_read_chunk_tree+0xef/0x2170 fs/btrfs/volumes.c:7462
> >         open_ctree+0x17f2/0x3a10 fs/btrfs/disk-io.c:3458
> >         btrfs_fill_super fs/btrfs/super.c:984 [inline]
> >         btrfs_get_tree_super fs/btrfs/super.c:1922 [inline]
> >         btrfs_get_tree_subvol fs/btrfs/super.c:2059 [inline]
> >         btrfs_get_tree+0xc6f/0x12d0 fs/btrfs/super.c:2093
> >         vfs_get_tree+0x8f/0x2b0 fs/super.c:1804
> >         do_new_mount+0x24a/0xa40 fs/namespace.c:3902
> >         do_mount fs/namespace.c:4239 [inline]
> >         __do_sys_mount fs/namespace.c:4450 [inline]
> >         __se_sys_mount+0x317/0x410 fs/namespace.c:4427
> >         do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >         do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> >         entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >=20
> > other info that might help us debug this:
> >=20
> >   Possible unsafe locking scenario:
> >=20
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(&type->s_umount_key#41/1);
> >                                 lock(uuid_mutex);
> >                                 lock(&type->s_umount_key#41/1);
> >    lock(uuid_mutex);
> >=20
> >   *** DEADLOCK ***
> >=20
> > Fixes: 7aacdf6feed1 ("btrfs: delay btrfs_open_devices() until super bloc=
> k is created")
> > Reported-by: syzbot+fa90fcaa28f5cd4b1fc1@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=3Dfa90fcaa28f5cd4b1fc1
> > Tested-by: syzbot+fa90fcaa28f5cd4b1fc1@syzkaller.appspotmail.com
> > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> > ---
> >   fs/btrfs/super.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >=20
> > diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> > index 237e60b53192..c2ce1eb53ad7 100644
> > --- a/fs/btrfs/super.c
> > +++ b/fs/btrfs/super.c
> > @@ -1864,11 +1864,10 @@ static int btrfs_get_tree_super(struct fs_contex=
> t *fc)
> >   	fs_devices =3D device->fs_devices;
> >   	fs_info->fs_devices =3D fs_devices;
> >  =20
> > +	mutex_unlock(&uuid_mutex);
> 
> No, you can not unlock uuid_mutex without opening the devices.
> 
> Just run the test case generic/604.
> 
> >   	sb =3D sget_fc(fc, btrfs_fc_test_super, set_anon_super_fc);
> > -	if (IS_ERR(sb)) {
> > -		mutex_unlock(&uuid_mutex);
> > +	if (IS_ERR(sb))
> >   		return PTR_ERR(sb);
> > -	}
> >  =20
> >   	set_device_specific_options(fs_info);
> >  =20
> > @@ -1887,6 +1886,7 @@ static int btrfs_get_tree_super(struct fs_context =
> *fc)
> >   		 * But the fs_info->fs_devices is not opened, we should not let
> >   		 * btrfs_free_fs_context() to close them.
> >   		 */
> > +		mutex_lock(&uuid_mutex);
> >   		fs_info->fs_devices =3D NULL;
> >   		mutex_unlock(&uuid_mutex);
> >  =20
> > @@ -1906,6 +1906,7 @@ static int btrfs_get_tree_super(struct fs_context =
> *fc)
> >   		 */
> >   		ASSERT(fc->s_fs_info =3D=3D NULL);
> >  =20
> > +		mutex_lock(&uuid_mutex);
> >   		ret =3D btrfs_open_devices(fs_devices, mode, sb);
> >   		mutex_unlock(&uuid_mutex);
> >   		if (ret < 0) {