btrfs: remove unnecessary logic when running new delayed references

When running delayed references, at btrfs_run_delayed_refs(), we have this
logic to run any new delayed references that might have been added just
after we ran all delayed references. This logic grabs the first delayed
reference, then locks it to wait for any contention on it before running
all new delayed references. This however is pointless and not necessary
because at __btrfs_run_delayed_refs() when we start running delayed
references, we pick the first reference with btrfs_obtain_ref_head() and
then we will lock it (with btrfs_delayed_ref_lock()).

So remove the duplicate and unnecessary logic at btrfs_run_delayed_refs().
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2137,9 +2137,7 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 			   unsigned long count)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
-	struct rb_node *node;
 	struct btrfs_delayed_ref_root *delayed_refs;
-	struct btrfs_delayed_ref_head *head;
 	int ret;
 	int run_all = count == (unsigned long)-1;
 
@@ -2168,25 +2166,16 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 		btrfs_create_pending_block_groups(trans);
 
 		spin_lock(&delayed_refs->lock);
-		node = rb_first_cached(&delayed_refs->href_root);
-		if (!node) {
+		if (RB_EMPTY_ROOT(&delayed_refs->href_root.rb_root)) {
 			spin_unlock(&delayed_refs->lock);
-			goto out;
+			return 0;
 		}
-		head = rb_entry(node, struct btrfs_delayed_ref_head,
-				href_node);
-		refcount_inc(&head->refs);
 		spin_unlock(&delayed_refs->lock);
 
-		/* Mutex was contended, block until it's released and retry. */
-		mutex_lock(&head->mutex);
-		mutex_unlock(&head->mutex);
-
-		btrfs_put_delayed_ref_head(head);
 		cond_resched();
 		goto again;
 	}
-out:
+
 	return 0;
 }