btrfs: bail out of uuid tree scanning if we're closing

In doing my fsstress+EIO stress testing I started running into issues
where umount would get stuck forever because the uuid checker was
chewing through the thousands of subvolumes I had created.

We shouldn't block umount on this, simply bail if we're unmounting the
fs.  We need to make sure we don't mark the UUID tree as ok, so we only
set that bit if we made it through the whole rescan operation, but
otherwise this is completely safe.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2847,7 +2847,9 @@ static int btrfs_uuid_rescan_kthread(void *data)
 	 */
 	ret = btrfs_uuid_tree_iterate(fs_info);
 	if (ret < 0) {
-		btrfs_warn(fs_info, "iterating uuid_tree failed %d", ret);
+		if (ret != -EINTR)
+			btrfs_warn(fs_info, "iterating uuid_tree failed %d",
+				   ret);
 		up(&fs_info->uuid_tree_rescan_sem);
 		return ret;
 	}

fs/btrfs/uuid-tree.c

@@ -322,6 +322,10 @@ int btrfs_uuid_tree_iterate(struct btrfs_fs_info *fs_info)
 	}
 
 	while (1) {
+		if (btrfs_fs_closing(fs_info)) {
+			ret = -EINTR;
+			goto out;
+		}
 		cond_resched();
 		leaf = path->nodes[0];
 		slot = path->slots[0];

fs/btrfs/volumes.c

@@ -4325,6 +4325,7 @@ int btrfs_uuid_scan_kthread(void *data)
 	struct btrfs_root_item root_item;
 	u32 item_size;
 	struct btrfs_trans_handle *trans = NULL;
+	bool closing = false;
 
 	path = btrfs_alloc_path();
 	if (!path) {
@@ -4337,6 +4338,10 @@ int btrfs_uuid_scan_kthread(void *data)
 	key.offset = 0;
 
 	while (1) {
+		if (btrfs_fs_closing(fs_info)) {
+			closing = true;
+			break;
+		}
 		ret = btrfs_search_forward(root, &key, path,
 				BTRFS_OLDEST_GENERATION);
 		if (ret) {
@@ -4436,7 +4441,7 @@ int btrfs_uuid_scan_kthread(void *data)
 		btrfs_end_transaction(trans);
 	if (ret)
 		btrfs_warn(fs_info, "btrfs_uuid_scan_kthread failed %d", ret);
-	else
+	else if (!closing)
 		set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
 	up(&fs_info->uuid_tree_rescan_sem);
 	return 0;