Commit 80eb65cc authored Sep 08, 2025 by Filipe Manana Committed by David Sterba Sep 15, 2025

btrfs: annotate block group access with data_race() when sorting for reclaim

When sorting the block group list for reclaim we are using a block group's
used bytes counter without taking the block group's spinlock, so we can
race with a concurrent task updating it (at btrfs_update_block_group()),
which makes tools like KCSAN unhappy and report a race.

Since the sorting is not strictly needed from a functional perspective
and such races should rarely cause any ordering changes (only load/store
tearing could cause them), not to mention that after the sorting the
ordering may no longer be accurate due to concurrent allocations and
deallocations of extents in a block group, annotate the accesses to the
used counter with data_race() to silence KCSAN and similar tools.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

parent 8679d268

fs/btrfs/block-group.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -1795,7 +1795,14 @@ static int reclaim_bgs_cmp(void unused, const struct list_head a,
		bg1 = list_entry(a, struct btrfs_block_group, bg_list);
		bg2 = list_entry(b, struct btrfs_block_group, bg_list);

		return bg1->used > bg2->used;
		/*
		* Some other task may be updating the ->used field concurrently, but it
		* is not serious if we get a stale value or load/store tearing issues,
		* as sorting the list of block groups to reclaim is not critical and an
		* occasional imperfect order is ok. So silence KCSAN and avoid the
		* overhead of locking or any other synchronization.
		*/
		return data_race(bg1->used > bg2->used);
		}

		static inline bool btrfs_should_reclaim(const struct btrfs_fs_info *fs_info)