[PATCH] fs/btrfs: Use atomic_try_cmpxchg in free_extent_buffer

From: Uros Bizjak
Date: Tue Aug 09 2022 - 12:37:31 EST


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.

Cc: Chris Mason <clm@xxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
fs/btrfs/extent_io.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index bfae67c593c5..15ff196cbd6d 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -6328,18 +6328,16 @@ 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;
}

--
2.37.1