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btrfs: use atomic_try_cmpxchg in free_extent_buffer

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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>
This commit is contained in:
Uros Bizjak 2022-08-09 18:36:33 +02:00 committed by David Sterba
parent fc65bb5318
commit e5677f0560
1 changed files with 2 additions and 4 deletions
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6
fs/btrfs/extent_io.c
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@ -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;
while (1) {
refs = atomic_read(&eb->refs);
while (1) {
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;
}