bcachefs: Use try_cmpxchg() family of functions instead of cmpxchg()

Use try_cmpxchg() family of functions instead of
cmpxchg (*ptr, old, new) == old. x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, try_cmpxchg() implicitly assigns old *ptr value to "old" when
cmpxchg fails. There is no need to re-read the value in the loop.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 5 insertions, 6 deletions

diff --git a/fs/bcachefs/two_state_shared_lock.h b/fs/bcachefs/two_state_shared_lock.h
index 905801772002..7f647846b511 100644
--- a/fs/bcachefs/two_state_shared_lock.h
+++ b/fs/bcachefs/two_state_shared_lock.h
@@ -36,15 +36,14 @@ static inline void bch2_two_state_unlock(two_state_lock_t *lock, int s)
 static inline bool bch2_two_state_trylock(two_state_lock_t *lock, int s)
 {
 	long i = s ? 1 : -1;
-	long v = atomic_long_read(&lock->v), old;
+	long old;
 
+	old = atomic_long_read(&lock->v);
 	do {
-		old = v;
-
-		if (i > 0 ? v < 0 : v > 0)
+		if (i > 0 ? old < 0 : old > 0)
 			return false;
-	} while ((v = atomic_long_cmpxchg_acquire(&lock->v,
-					old, old + i)) != old);
+	} while (!atomic_long_try_cmpxchg_acquire(&lock->v, &old, old + i));
+
 	return true;
 }