Btrfs: fix lockdep warning about log_mutex

While checking INODE_REF/INODE_EXTREF for a corner case, we may acquire a
different inode's log_mutex with holding the current inode's log_mutex, and
lockdep has complained this with a possilble deadlock warning.

Fix this by using mutex_lock_nested() when processing the other inode's
log_mutex.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -37,6 +37,7 @@
  */
 #define LOG_INODE_ALL 0
 #define LOG_INODE_EXISTS 1
+#define LOG_OTHER_INODE 2
 
 /*
  * directory trouble cases
@@ -4641,7 +4642,7 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 	if (S_ISDIR(inode->i_mode) ||
 	    (!test_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
 		       &BTRFS_I(inode)->runtime_flags) &&
-	     inode_only == LOG_INODE_EXISTS))
+	     inode_only >= LOG_INODE_EXISTS))
 		max_key.type = BTRFS_XATTR_ITEM_KEY;
 	else
 		max_key.type = (u8)-1;
@@ -4665,7 +4666,13 @@ static int btrfs_log_inode(struct btrfs_trans_handle *trans,
 		return ret;
 	}
 
-	mutex_lock(&BTRFS_I(inode)->log_mutex);
+	if (inode_only == LOG_OTHER_INODE) {
+		inode_only = LOG_INODE_EXISTS;
+		mutex_lock_nested(&BTRFS_I(inode)->log_mutex,
+				  SINGLE_DEPTH_NESTING);
+	} else {
+		mutex_lock(&BTRFS_I(inode)->log_mutex);
+	}
 
 	/*
 	 * a brute force approach to making sure we get the most uptodate
@@ -4817,7 +4824,7 @@ again:
 				 * unpin it.
 				 */
 				err = btrfs_log_inode(trans, root, other_inode,
-						      LOG_INODE_EXISTS,
+						      LOG_OTHER_INODE,
 						      0, LLONG_MAX, ctx);
 				iput(other_inode);
 				if (err)