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 <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/block-group.c

···
 	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)