btrfs: annotate lockless read of defrag_bytes in should_nocow()

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: 47059d930f ("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>

fs/btrfs/inode.c

@@ -2293,7 +2293,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *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 (data_race(inode->defrag_bytes) &&
 		    btrfs_test_range_bit_exists(&inode->io_tree, start, end, EXTENT_DEFRAG))
 			return false;
 		return true;