should_nocow() reads inode->defrag_bytes without holding inode->lock, while btrfs_set_delalloc_extent() and btrfs_clear_delalloc_extent() update it under that spinlock. This is a data race. The read is a quick check used to decide whether to fall back to COW for a NOCOW inode: if defrag_bytes is non-zero and the range is tagged EXTENT_DEFRAG, we force COW so that defragmentation can rewrite the extent. Reading a stale value is harmless because: - A missed increment may skip COW once, but the defrag pass will redo the extent later. - A stale non-zero may force an unnecessary COW, which is a minor efficiency loss, not a correctness issue. On 64-bit platforms an aligned u64 load is naturally atomic so tearing cannot happen. On 32-bit platforms u64 may tear, but we only test for zero vs non-zero, so the heuristic stays correct regardless. Use data_race() annotation. Fixes: 47059d930f0e ("Btrfs: make defragment work with nodatacow option") Signed-off-by: Cen Zhang <zzzccc427@gmail.com> [ Use data_race() instead of READ_ONCXE() ] Signed-off-by: David Sterba <dsterba@suse.com>