[tip: locking/core] jump_label: Use atomic_try_cmpxchg() in static_key_slow_inc_cpuslocked()

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     d0c006402e7941558e5283ae434e2847c7999378
Gitweb:        https://git.kernel.org/tip/d0c006402e7941558e5283ae434e2847c7999378
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Wed, 19 Oct 2022 16:08:50 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 27 Oct 2022 10:35:41 +02:00

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 <ubizjak@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20221019140850.3395-1-ubizjak@xxxxxxxxx
---
 kernel/jump_label.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 714ac4c..4d6c6f5 100644
--- a/kernel/jump_label.c
+++ b/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) {