From 80eb65ccf6f72dc37b972583fe71cd8a50ff7e51 Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Mon, 8 Sep 2025 12:51:11 +0100 Subject: [PATCH] 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 Reviewed-by: Johannes Thumshirn Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/block-group.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c index 9bf282d2453c..499a9edf0ca3 100644 --- a/fs/btrfs/block-group.c +++ b/fs/btrfs/block-group.c @@ -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) -- 2.25.1