Since commit bafc9b754f75 ("vfs: More precise tests in d_invalidate"),
mounted subvolumes can be deleted because d_invalidate() won't fail.
However, we run into problems when we attempt to delete the default
subvolume while it is mounted as the root filesystem:

        # btrfs subvol list /
        ID 257 gen 306 top level 5 path rootvol
        ID 267 gen 334 top level 5 path snap1
        # btrfs subvol get-default /
        ID 267 gen 334 top level 5 path snap1
        # btrfs inspect-internal rootid /
        267
        # mount -o subvol=/ /dev/vda1 /mnt
        # btrfs subvol del /mnt/snap1
        Delete subvolume (no-commit): '/mnt/snap1'
        ERROR: cannot delete '/mnt/snap1' - Operation not permitted
        # findmnt /
        findmnt: can't read /proc/mounts: No such file or directory
        # ls /proc
        #

Markus reported that this same scenario simply led to a kernel oops.

This happens because in btrfs_ioctl_snap_destroy(), we call
d_invalidate() before we check may_destroy_subvol(), which means that we
detach the submounts and drop the dentry before erroring out. Instead,
we should only invalidate the dentry once we know that we're going
through with the deletion.

Cc: <redacted>
Fixes: bafc9b754f75 ("vfs: More precise tests in d_invalidate")
Reported-by: Markus Schauler <redacted>
Signed-off-by: Omar Sandoval <osandov@osandov.com>
---
The other fix for preventing all mounted subvolumes from being deleted
would preclude this, but it sounded like we were leaning towards
enforcing that in userspace once subvolume info becomes available in
/proc/mounts, so this should be fixed separately.

 fs/btrfs/ioctl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 1c22c6518504..8edb8544088b 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c

@@ -2413,14 +2413,14 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
                goto out_unlock_inode;
        }

-       d_invalidate(dentry);
-
        down_write(&root->fs_info->subvol_sem);

        err = may_destroy_subvol(dest);
        if (err)
                goto out_up_write;

+       d_invalidate(dentry);
+