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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. Add READ_ONCE() to prevent the compiler from caching or splitting the load and to document the intentional lock-free pattern. Fixes: 47059d930f0e ("Btrfs: make defragment work with nodatacow option") Signed-off-by: Cen Zhang --- fs/btrfs/inode.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index a6da98435ef7..afc5d75d2dcb 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -2420,7 +2420,7 @@ static noinline int run_delalloc_nocow(struct btrfs_i= node *inode, static bool should_nocow(struct btrfs_inode *inode, u64 start, u64 end) { if (inode->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)) { - if (inode->defrag_bytes && + if (READ_ONCE(inode->defrag_bytes) && btrfs_test_range_bit_exists(&inode->io_tree, start, end, EXTENT_DEFR= AG)) return false; return true; --=20 2.34.1