btrfs: Add check to avoid cleanup roots already in fs_info->dead_roots.

Current btrfs_orphan_cleanup will also cleanup roots which is already in
fs_info->dead_roots without protection.
This will have conditional race with fs_info->cleaner_kthread.

This patch will use refs in root->root_item to detect roots in
dead_roots and avoid conflicts.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/disk-io.c

@@ -3534,28 +3534,51 @@ int btrfs_cleanup_fs_roots(struct btrfs_fs_info *fs_info)
 {
 	u64 root_objectid = 0;
 	struct btrfs_root *gang[8];
-	int i;
-	int ret;
+	int i = 0;
+	int err = 0;
+	unsigned int ret = 0;
+	int index;
 
 	while (1) {
+		index = srcu_read_lock(&fs_info->subvol_srcu);
 		ret = radix_tree_gang_lookup(&fs_info->fs_roots_radix,
 					     (void **)gang, root_objectid,
 					     ARRAY_SIZE(gang));
-		if (!ret)
+		if (!ret) {
+			srcu_read_unlock(&fs_info->subvol_srcu, index);
 			break;
-
+		}
 		root_objectid = gang[ret - 1]->root_key.objectid + 1;
+
 		for (i = 0; i < ret; i++) {
-			int err;
+			/* Avoid to grab roots in dead_roots */
+			if (btrfs_root_refs(&gang[i]->root_item) == 0) {
+				gang[i] = NULL;
+				continue;
+			}
+			/* grab all the search result for later use */
+			gang[i] = btrfs_grab_fs_root(gang[i]);
+		}
+		srcu_read_unlock(&fs_info->subvol_srcu, index);
 
+		for (i = 0; i < ret; i++) {
+			if (!gang[i])
+				continue;
 			root_objectid = gang[i]->root_key.objectid;
 			err = btrfs_orphan_cleanup(gang[i]);
 			if (err)
-				return err;
+				break;
+			btrfs_put_fs_root(gang[i]);
 		}
 		root_objectid++;
 	}
-	return 0;
+
+	/* release the uncleaned roots due to error */
+	for (; i < ret; i++) {
+		if (gang[i])
+			btrfs_put_fs_root(gang[i]);
+	}
+	return err;
 }
 
 int btrfs_commit_super(struct btrfs_root *root)