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jump_label: Use atomic_try_cmpxchg() in static_key_slow_inc_cpuslocked()

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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
This commit is contained in:
Uros Bizjak 2022-10-19 16:08:50 +02:00 committed by Peter Zijlstra
parent 247f34f7b8
commit d0c006402e
1 changed files with 2 additions and 6 deletions
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8
kernel/jump_label.c
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@ -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) {