From: Boris Burkov boris@bur.io
[ Upstream commit 7511e29cf1355b2c47d0effb39e463119913e2f6 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/extent-tree.c | 8 ++++++++ fs/btrfs/transaction.c | 12 ++++++++++++ 2 files changed, 20 insertions(+)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 3014a1a23efdb..6d615711f0400 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -2874,7 +2874,15 @@ 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);
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index aca83a98b75a2..c0e9d4bbe380d 100644 --- a/fs/btrfs/transaction.c +++ b/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); } }