2024-06-10 12:56:44

by Thomas Gleixner

Subject: [patch 4/4] jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

Make the code more obvious and add proper comments to avoid future head
scratching.

Signed-off-by: Thomas Gleixner <[email protected]>
---
kernel/jump_label.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)

--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -162,22 +162,24 @@ bool static_key_slow_inc_cpuslocked(stru
if (static_key_fast_inc_not_disabled(key))
return true;

- jump_label_lock();
- if (atomic_read(&key->enabled) == 0) {
- atomic_set(&key->enabled, -1);
+ guard(mutex)(&jump_label_mutex);
+ /* Try to mark it as 'enabling in progress. */
+ if (!atomic_cmpxchg(&key->enabled, 0, -1)) {
jump_label_update(key);
/*
- * Ensure that if the above cmpxchg loop observes our positive
- * value, it must also observe all the text changes.
+ * Ensure that when static_key_fast_inc_not_disabled() or
+ * static_key_slow_try_dec() observe the positive value,
+ * they must also observe all the text changes.
*/
atomic_set_release(&key->enabled, 1);
} else {
- if (WARN_ON_ONCE(!static_key_fast_inc_not_disabled(key))) {
- jump_label_unlock();
+ /*
+ * While holding the mutex this should never observe
+ * anything else than a value >= 1 and succeed
+ */
+ if (WARN_ON_ONCE(!static_key_fast_inc_not_disabled(key)))
return false;
- }
}
- jump_label_unlock();
return true;
}

2024-06-12 14:00:56

by Uros Bizjak

Subject: Re: [patch 4/4] jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

> Make the code more obvious and add proper comments to avoid future head
> scratching.
>
> Signed-off-by: Thomas Gleixner <[email protected]>
> ---
> kernel/jump_label.c | 20 +++++++++++---------
> 1 file changed, 11 insertions(+), 9 deletions(-)
>
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -162,22 +162,24 @@ bool static_key_slow_inc_cpuslocked(stru
> if (static_key_fast_inc_not_disabled(key))
> return true;
>
> - jump_label_lock();
> - if (atomic_read(&key->enabled) == 0) {
> - atomic_set(&key->enabled, -1);
> + guard(mutex)(&jump_label_mutex);
> + /* Try to mark it as 'enabling in progress. */

Missing closing quotation mark above.

> + if (!atomic_cmpxchg(&key->enabled, 0, -1)) {

This can be:

int tmp = 0;
if (atomic_try_cmpxchg(&key->enabled, &tmp, -1)) {
...

and will result in more optimal code and will IMO also be more readable
because it is clear that the code is executed when cmpxchg succeeds.

(BTW: The atomic_read()/atomic_set() pair can also be converted to
cmpxchg in static_key_enable_cpuslocked().)

Uros.