Whenever the check for a send in progress introduced in commit
521e0546c970 (btrfs: protect snapshots from deleting during send) is
hit, we return without unlocking inode->i_mutex. This is easy to see
with lockdep enabled:

[  +0.000059] ================================================
[  +0.000028] [ BUG: lock held when returning to user space! ]
[  +0.000029] 4.0.0-rc5-00096-g3c435c1 #93 Not tainted
[  +0.000026] ------------------------------------------------
[  +0.000029] btrfs/211 is leaving the kernel with locks still held!
[  +0.000029] 1 lock held by btrfs/211:
[  +0.000023]  #0:  (&type->i_mutex_dir_key){+.+.+.}, at: [<ffffffff8135b8df>] 
btrfs_ioctl_snap_destroy+0x2df/0x7a0

Make sure we unlock it in the error path.

Reviewed-by: Filipe Manana <fdman...@suse.com>
Reviewed-by: David Sterba <dste...@suse.cz>
Cc: sta...@vger.kernel.org
Signed-off-by: Omar Sandoval <osan...@osandov.com>
---
Just resending this with Filipe's and David's Reviewed-bys and Cc-ing
stable.

 fs/btrfs/ioctl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 74609b9..9fde01f 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2403,7 +2403,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file 
*file,
                        "Attempt to delete subvolume %llu during send",
                        dest->root_key.objectid);
                err = -EPERM;
-               goto out_dput;
+               goto out_unlock_inode;
        }
 
        d_invalidate(dentry);
@@ -2498,6 +2498,7 @@ out_up_write:
                                root_flags & ~BTRFS_ROOT_SUBVOL_DEAD);
                spin_unlock(&dest->root_item_lock);
        }
+out_unlock_inode:
        mutex_unlock(&inode->i_mutex);
        if (!err) {
                shrink_dcache_sb(root->fs_info->sb);
-- 
2.3.5

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