Btrfs: add ioctl to wait for qgroup rescan completion

btrfs_qgroup_wait_for_completion waits until the currently running qgroup
operation completes. It returns immediately when no rescan process is in
progress. This is useful to automate things around the rescan process (e.g.
testing).

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/qgroup.c

@@ -2070,6 +2070,8 @@ out:
 	} else {
 		pr_err("btrfs: qgroup scan failed with %d\n", err);
 	}
+
+	complete_all(&fs_info->qgroup_rescan_completion);
 }
 
 static void
@@ -2110,6 +2112,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
+	init_completion(&fs_info->qgroup_rescan_completion);
 
 	/* clear all current qgroup tracking information */
 	for (n = rb_first(&fs_info->qgroup_tree); n; n = rb_next(n)) {
@@ -2126,3 +2129,21 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	return 0;
 }
+
+int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info)
+{
+	int running;
+	int ret = 0;
+
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	spin_lock(&fs_info->qgroup_lock);
+	running = fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	spin_unlock(&fs_info->qgroup_lock);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	if (running)
+		ret = wait_for_completion_interruptible(
+					&fs_info->qgroup_rescan_completion);
+
+	return ret;
+}