btrfs: fix lockdep splat in btrfs_recover_relocation

While testing the error paths of relocation I hit the following lockdep
splat:

  ======================================================
  WARNING: possible circular locking dependency detected
  5.10.0-rc6+ Freescale#217 Not tainted
  ------------------------------------------------------
  mount/779 is trying to acquire lock:
  ffffa0e676945418 (&fs_info->balance_mutex){+.+.}-{3:3}, at: btrfs_recover_balance+0x2f0/0x340

  but task is already holding lock:
  ffffa0e60ee31da8 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x100

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> Freescale#2 (btrfs-root-00){++++}-{3:3}:
	 down_read_nested+0x43/0x130
	 __btrfs_tree_read_lock+0x27/0x100
	 btrfs_read_lock_root_node+0x31/0x40
	 btrfs_search_slot+0x462/0x8f0
	 btrfs_update_root+0x55/0x2b0
	 btrfs_drop_snapshot+0x398/0x750
	 clean_dirty_subvols+0xdf/0x120
	 btrfs_recover_relocation+0x534/0x5a0
	 btrfs_start_pre_rw_mount+0xcb/0x170
	 open_ctree+0x151f/0x1726
	 btrfs_mount_root.cold+0x12/0xea
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 vfs_kern_mount.part.0+0x71/0xb0
	 btrfs_mount+0x10d/0x380
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 path_mount+0x433/0xc10
	 __x64_sys_mount+0xe3/0x120
	 do_syscall_64+0x33/0x40
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> Freescale#1 (sb_internal#2){.+.+}-{0:0}:
	 start_transaction+0x444/0x700
	 insert_balance_item.isra.0+0x37/0x320
	 btrfs_balance+0x354/0xf40
	 btrfs_ioctl_balance+0x2cf/0x380
	 __x64_sys_ioctl+0x83/0xb0
	 do_syscall_64+0x33/0x40
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> #0 (&fs_info->balance_mutex){+.+.}-{3:3}:
	 __lock_acquire+0x1120/0x1e10
	 lock_acquire+0x116/0x370
	 __mutex_lock+0x7e/0x7b0
	 btrfs_recover_balance+0x2f0/0x340
	 open_ctree+0x1095/0x1726
	 btrfs_mount_root.cold+0x12/0xea
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 vfs_kern_mount.part.0+0x71/0xb0
	 btrfs_mount+0x10d/0x380
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 path_mount+0x433/0xc10
	 __x64_sys_mount+0xe3/0x120
	 do_syscall_64+0x33/0x40
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  other info that might help us debug this:

  Chain exists of:
    &fs_info->balance_mutex --> sb_internal#2 --> btrfs-root-00

   Possible unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(btrfs-root-00);
				 lock(sb_internal#2);
				 lock(btrfs-root-00);
    lock(&fs_info->balance_mutex);

   *** DEADLOCK ***

  2 locks held by mount/779:
   #0: ffffa0e60dc040e0 (&type->s_umount_key#47/1){+.+.}-{3:3}, at: alloc_super+0xb5/0x380
   Freescale#1: ffffa0e60ee31da8 (btrfs-root-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x27/0x100

  stack backtrace:
  CPU: 0 PID: 779 Comm: mount Not tainted 5.10.0-rc6+ Freescale#217
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
  Call Trace:
   dump_stack+0x8b/0xb0
   check_noncircular+0xcf/0xf0
   ? trace_call_bpf+0x139/0x260
   __lock_acquire+0x1120/0x1e10
   lock_acquire+0x116/0x370
   ? btrfs_recover_balance+0x2f0/0x340
   __mutex_lock+0x7e/0x7b0
   ? btrfs_recover_balance+0x2f0/0x340
   ? btrfs_recover_balance+0x2f0/0x340
   ? rcu_read_lock_sched_held+0x3f/0x80
   ? kmem_cache_alloc_trace+0x2c4/0x2f0
   ? btrfs_get_64+0x5e/0x100
   btrfs_recover_balance+0x2f0/0x340
   open_ctree+0x1095/0x1726
   btrfs_mount_root.cold+0x12/0xea
   ? rcu_read_lock_sched_held+0x3f/0x80
   legacy_get_tree+0x30/0x50
   vfs_get_tree+0x28/0xc0
   vfs_kern_mount.part.0+0x71/0xb0
   btrfs_mount+0x10d/0x380
   ? __kmalloc_track_caller+0x2f2/0x320
   legacy_get_tree+0x30/0x50
   vfs_get_tree+0x28/0xc0
   ? capable+0x3a/0x60
   path_mount+0x433/0xc10
   __x64_sys_mount+0xe3/0x120
   do_syscall_64+0x33/0x40
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

This is straightforward to fix, simply release the path before we setup
the balance_ctl.

CC: [email protected] # 4.4+
Reviewed-by: Qu Wenruo <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

fs/btrfs/volumes.c

@@ -4318,6 +4318,8 @@ int btrfs_recover_balance(struct btrfs_fs_info *fs_info)
 		btrfs_warn(fs_info,
 	"balance: cannot set exclusive op status, resume manually");
 
+	btrfs_release_path(path);
+
 	mutex_lock(&fs_info->balance_mutex);
 	BUG_ON(fs_info->balance_ctl);
 	spin_lock(&fs_info->balance_lock);