I got the following lockdep splat while testing seed devices

======================================================
WARNING: possible circular locking dependency detected
5.14.0-rc2+ #409 Not tainted
------------------------------------------------------
mount/34004 is trying to acquire lock:
ffff9eaac48188e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: clone_fs_devices+0x4d/0x170

but task is already holding lock:
ffff9eaac766d438 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x100

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (btrfs-chunk-00){++++}-{3:3}:
        down_read_nested+0x46/0x60
        __btrfs_tree_read_lock+0x24/0x100
        btrfs_read_lock_root_node+0x31/0x40
        btrfs_search_slot+0x480/0x930
        btrfs_update_device+0x63/0x180
        btrfs_chunk_alloc_add_chunk_item+0xdc/0x3a0
        btrfs_chunk_alloc+0x281/0x540
        find_free_extent+0x10ca/0x1790
        btrfs_reserve_extent+0xbf/0x1d0
        btrfs_alloc_tree_block+0xb1/0x320
        __btrfs_cow_block+0x136/0x5f0
        btrfs_cow_block+0x107/0x210
        btrfs_search_slot+0x56a/0x930
        btrfs_truncate_inode_items+0x187/0xef0
        btrfs_truncate_free_space_cache+0x11c/0x210
        delete_block_group_cache+0x6f/0xb0
        btrfs_relocate_block_group+0xf8/0x350
        btrfs_relocate_chunk+0x38/0x120
        btrfs_balance+0x79b/0xf00
        btrfs_ioctl_balance+0x327/0x400
        __x64_sys_ioctl+0x80/0xb0
        do_syscall_64+0x38/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}:
        __mutex_lock+0x7d/0x750
        btrfs_init_new_device+0x6d6/0x1540
        btrfs_ioctl+0x1b12/0x2d30
        __x64_sys_ioctl+0x80/0xb0
        do_syscall_64+0x38/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #0 (&fs_devs->device_list_mutex){+.+.}-{3:3}:
        __lock_acquire+0x10ea/0x1d90
        lock_acquire+0xb5/0x2b0
        __mutex_lock+0x7d/0x750
        clone_fs_devices+0x4d/0x170
        btrfs_read_chunk_tree+0x32f/0x800
        open_ctree+0xae3/0x16f0
        btrfs_mount_root.cold+0x12/0xea
        legacy_get_tree+0x2d/0x50
        vfs_get_tree+0x25/0xc0
        vfs_kern_mount.part.0+0x71/0xb0
        btrfs_mount+0x10d/0x380
        legacy_get_tree+0x2d/0x50
        vfs_get_tree+0x25/0xc0
        path_mount+0x433/0xb60
        __x64_sys_mount+0xe3/0x120
        do_syscall_64+0x38/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:

Chain exists of:
   &fs_devs->device_list_mutex --> &fs_info->chunk_mutex --> btrfs-chunk-00

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(btrfs-chunk-00);
                                lock(&fs_info->chunk_mutex);
                                lock(btrfs-chunk-00);
   lock(&fs_devs->device_list_mutex);

  *** DEADLOCK ***

3 locks held by mount/34004:
  #0: ffff9eaad75c00e0 (&type->s_umount_key#47/1){+.+.}-{3:3}, at: alloc_super+0xd5/0x3b0
  #1: ffffffffbd2dcf08 (uuid_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x59/0x800
  #2: ffff9eaac766d438 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x100

stack backtrace:
CPU: 0 PID: 34004 Comm: mount Not tainted 5.14.0-rc2+ #409
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
Call Trace:
  dump_stack_lvl+0x57/0x72
  check_noncircular+0xcf/0xf0
  __lock_acquire+0x10ea/0x1d90
  lock_acquire+0xb5/0x2b0
  ? clone_fs_devices+0x4d/0x170
  ? lock_is_held_type+0xa5/0x120
  __mutex_lock+0x7d/0x750
  ? clone_fs_devices+0x4d/0x170
  ? clone_fs_devices+0x4d/0x170
  ? lockdep_init_map_type+0x47/0x220
  ? debug_mutex_init+0x33/0x40
  clone_fs_devices+0x4d/0x170
  ? lock_is_held_type+0xa5/0x120
  btrfs_read_chunk_tree+0x32f/0x800
  ? find_held_lock+0x2b/0x80
  open_ctree+0xae3/0x16f0
  btrfs_mount_root.cold+0x12/0xea
  ? rcu_read_lock_sched_held+0x3f/0x80
  ? kfree+0x1f6/0x410
  legacy_get_tree+0x2d/0x50
  vfs_get_tree+0x25/0xc0
  vfs_kern_mount.part.0+0x71/0xb0
  btrfs_mount+0x10d/0x380
  ? kfree+0x1f6/0x410
  legacy_get_tree+0x2d/0x50
  vfs_get_tree+0x25/0xc0
  path_mount+0x433/0xb60
  __x64_sys_mount+0xe3/0x120
  do_syscall_64+0x38/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f6cbcd9788e

It is because we take the ->device_list_mutex in this path while holding
onto the tree locks in the chunk root.  However we do not need the lock
here, because we're already holding onto the uuid_mutex, and in fact
have removed all other uses of the ->device_list_mutex in this path
because of this.  Remove the ->device_list_mutex locking here, add an
assert for the uuid_mutex and the problem is fixed.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
  fs/btrfs/volumes.c | 5 ++---
  1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f622e93a6ff1..bdfcc35335c3 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -1000,11 +1000,12 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
  	struct btrfs_device *orig_dev;
  	int ret = 0;
  
+	lockdep_assert_held(&uuid_mutex);
+
  	fs_devices = alloc_fs_devices(orig->fsid, NULL);
  	if (IS_ERR(fs_devices))
  		return fs_devices;
  
-	mutex_lock(&orig->device_list_mutex);
  	fs_devices->total_devices = orig->total_devices;
  
  	list_for_each_entry(orig_dev, &orig->devices, dev_list) {

@@ -1036,10 +1037,8 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
  		device->fs_devices = fs_devices;
  		fs_devices->num_devices++;
  	}
-	mutex_unlock(&orig->device_list_mutex);
  	return fs_devices;
  error:
-	mutex_unlock(&orig->device_list_mutex);
  	free_fs_devices(fs_devices);
  	return ERR_PTR(ret);
  }