btrfs: harden block_group::bg_list against list_del() races

As far as I can tell, these calls of list_del_init() on bg_list cannot
run concurrently with btrfs_mark_bg_unused() or btrfs_mark_bg_to_reclaim(),
as they are in transaction error paths and situations where the block
group is readonly.

However, if there is any chance at all of racing with mark_bg_unused(),
or a different future user of bg_list, better to be safe than sorry.

Otherwise we risk the following interleaving (bg_list refcount in parens)

T1 (some random op)                       T2 (btrfs_mark_bg_unused)
                                        !list_empty(&bg->bg_list); (1)
list_del_init(&bg->bg_list); (1)
                                        list_move_tail (1)
btrfs_put_block_group (0)
                                        btrfs_delete_unused_bgs
                                             bg = list_first_entry
                                             list_del_init(&bg->bg_list);
                                             btrfs_put_block_group(bg); (-1)

Ultimately, this results in a broken ref count that hits zero one deref
early and the real final deref underflows the refcount, resulting in a WARNING.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: boryas

fs/btrfs/extent-tree.c

@@ -2868,7 +2868,15 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans)
 						   block_group->length,
 						   &trimmed);
 
+		/*
+		 * Not strictly necessary to lock, as the block_group should be
+		 * read-only from btrfs_delete_unused_bgs().
+		 */
+		ASSERT(block_group->ro);
+		spin_lock(&fs_info->unused_bgs_lock);
 		list_del_init(&block_group->bg_list);
+		spin_unlock(&fs_info->unused_bgs_lock);
+
 		btrfs_unfreeze_block_group(block_group);
 		btrfs_put_block_group(block_group);
 

fs/btrfs/transaction.c

@@ -160,7 +160,13 @@ void btrfs_put_transaction(struct btrfs_transaction *transaction)
 			cache = list_first_entry(&transaction->deleted_bgs,
 						 struct btrfs_block_group,
 						 bg_list);
+			/*
+			 * Not strictly necessary to lock, as no other task will be using a
+			 * block_group on the deleted_bgs list during a transaction abort.
+			 */
+			spin_lock(&transaction->fs_info->unused_bgs_lock);
 			list_del_init(&cache->bg_list);
+			spin_unlock(&transaction->fs_info->unused_bgs_lock);
 			btrfs_unfreeze_block_group(cache);
 			btrfs_put_block_group(cache);
 		}
@@ -2096,7 +2102,13 @@ static void btrfs_cleanup_pending_block_groups(struct btrfs_trans_handle *trans)
 
        list_for_each_entry_safe(block_group, tmp, &trans->new_bgs, bg_list) {
                btrfs_dec_delayed_refs_rsv_bg_inserts(fs_info);
+		/*
+		* Not strictly necessary to lock, as no other task will be using a
+		* block_group on the new_bgs list during a transaction abort.
+		*/
+	       spin_lock(&fs_info->unused_bgs_lock);
                list_del_init(&block_group->bg_list);
+	       spin_unlock(&fs_info->unused_bgs_lock);
        }
 }