Btrfs: fix deadlock between clone/dedupe and rename

Reflinking (clone/dedupe) and rename are operations that operate on two
inodes and therefore need to lock them in the same order to avoid ABBA
deadlocks. It happens that Btrfs' reflink implementation always locked
them in a different order from VFS's lock_two_nondirectories() helper,
which is used by the rename code in VFS, resulting in ABBA type deadlocks.

Btrfs' locking order:

  static void btrfs_double_inode_lock(struct inode *inode1, struct inode *inode2)
  {
         if (inode1 < inode2)
                swap(inode1, inode2);

         inode_lock_nested(inode1, I_MUTEX_PARENT);
         inode_lock_nested(inode2, I_MUTEX_CHILD);
  }

VFS's locking order:

  void lock_two_nondirectories(struct inode *inode1, struct inode *inode2)
  {
        if (inode1 > inode2)
                swap(inode1, inode2);

        if (inode1 && !S_ISDIR(inode1->i_mode))
                inode_lock(inode1);
        if (inode2 && !S_ISDIR(inode2->i_mode) && inode2 != inode1)
                inode_lock_nested(inode2, I_MUTEX_NONDIR2);
}

Fix this by killing the btrfs helper function that does the double inode
locking and replace it with VFS's helper lock_two_nondirectories().
Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Fixes: 416161db ("btrfs: offline dedupe")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3207,21 +3207,6 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
 	return ret;
 }
 
-static void btrfs_double_inode_unlock(struct inode *inode1, struct inode *inode2)
-{
-	inode_unlock(inode1);
-	inode_unlock(inode2);
-}
-
-static void btrfs_double_inode_lock(struct inode *inode1, struct inode *inode2)
-{
-	if (inode1 < inode2)
-		swap(inode1, inode2);
-
-	inode_lock_nested(inode1, I_MUTEX_PARENT);
-	inode_lock_nested(inode2, I_MUTEX_CHILD);
-}
-
 static void btrfs_double_extent_unlock(struct inode *inode1, u64 loff1,
 				       struct inode *inode2, u64 loff2, u64 len)
 {
@@ -3956,7 +3941,7 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	if (same_inode)
 		inode_lock(inode_in);
 	else
-		btrfs_double_inode_lock(inode_in, inode_out);
+		lock_two_nondirectories(inode_in, inode_out);
 
 	/* don't make the dst file partly checksummed */
 	if ((BTRFS_I(inode_in)->flags & BTRFS_INODE_NODATASUM) !=
@@ -4013,7 +3998,7 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 	if (same_inode)
 		inode_unlock(inode_in);
 	else
-		btrfs_double_inode_unlock(inode_in, inode_out);
+		unlock_two_nondirectories(inode_in, inode_out);
 
 	return ret;
 }
@@ -4043,7 +4028,7 @@ loff_t btrfs_remap_file_range(struct file *src_file, loff_t off,
 	if (same_inode)
 		inode_unlock(src_inode);
 	else
-		btrfs_double_inode_unlock(src_inode, dst_inode);
+		unlock_two_nondirectories(src_inode, dst_inode);
 
 	return ret < 0 ? ret : len;
 }