jump_label: Use atomic_try_cmpxchg() in static_key_slow_inc_cpuslocked()

Use atomic_try_cmpxchg() instead of atomic_cmpxchg (*ptr, old, new) ==
old in static_key_slow_inc_cpuslocked().  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, atomic_try_cmpxchg() implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: ubizjak

kernel/jump_label.c

@@ -115,8 +115,6 @@ EXPORT_SYMBOL_GPL(static_key_count);
 
 void static_key_slow_inc_cpuslocked(struct static_key *key)
 {
-	int v, v1;
-
 	STATIC_KEY_CHECK_USE(key);
 	lockdep_assert_cpus_held();
 
@@ -132,11 +130,9 @@ void static_key_slow_inc_cpuslocked(struct static_key *key)
 	 * so it counts as "enabled" in jump_label_update().  Note that
 	 * atomic_inc_unless_negative() checks >= 0, so roll our own.
 	 */
-	for (v = atomic_read(&key->enabled); v > 0; v = v1) {
-		v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
-		if (likely(v1 == v))
+	for (int v = atomic_read(&key->enabled); v > 0; )
+		if (likely(atomic_try_cmpxchg(&key->enabled, &v, v + 1)))
 			return;
-	}
 
 	jump_label_lock();
 	if (atomic_read(&key->enabled) == 0) {