btrfs: use atomic_try_cmpxchg in free_extent_buffer

Use `atomic_try_cmpxchg(ptr, &old, new)` instead of
`atomic_cmpxchg(ptr, old, new) == old` in free_extent_buffer. This
has two benefits:

- The x86 cmpxchg instruction returns success in the ZF flag, so this
  change saves a compare after cmpxchg, as well as a related move
  instruction in the front of cmpxchg.

- atomic_try_cmpxchg implicitly assigns the *ptr value to &old when
  cmpxchg fails, enabling further code simplifications.

This patch has no functional change.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index cf4f19e80e2f7f174ecdd245f4dd4b5e9d7dbd87..63decf3abe26ba4db5562cd711d053092d77d5f4 100644 (file)
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -6362,18 +6362,16 @@ static int release_extent_buffer(struct extent_buffer *eb)
 void free_extent_buffer(struct extent_buffer *eb)
 {
        int refs;
-       int old;
        if (!eb)
                return;
 
+       refs = atomic_read(&eb->refs);
        while (1) {
-               refs = atomic_read(&eb->refs);
                if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3)
                    || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) &&
                        refs == 1))
                        break;
-               old = atomic_cmpxchg(&eb->refs, refs, refs - 1);
-               if (old == refs)
+               if (atomic_try_cmpxchg(&eb->refs, &refs, refs - 1))
                        return;
        }