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@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221019140850.3395-1-ubizjak@gmail.com

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 714ac4c3b556de14ab896f45d6d9a905552bb118..4d6c6f5f60db83f415b8ee0d56fcc5bda2a15650 100644 (file)
--- 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) {