Re: [PATCH v4 next 0/9] locking/osq_lock: Optimisations to osq_lock code

[Haakon Bugge <haakon.bugge@oracle.com>]

> On 9 Sep 2026, at 22:33, Waiman Long <longman@redhat.com> wrote:

[snip]

> > Could you make that change to the existing code and rerun the test 
> > again on arm64 to see if it can pass? 
> 
> osq_lock/unlock() is special in the sense that lock transfer can happen 
> either in the lock cacheline or the node->locked cacheline. Try the 
> patch below to see if it helps to pass the test.
> 
> Thanks,
> Longman
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index b4233dc2c2b0..51cecf297692 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -143,7 +143,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>          * is implemented with a monitor-wait. vcpu_is_preempted() 
> relies on
>          * polling, be careful.
>          */
> -       if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
> +       if (smp_cond_load_acquire(&node->locked, VAL || need_resched() ||
> vcpu_is_preempted(node_cpu(node->prev))))
>                 return true;
> 
> @@ -224,11 +224,11 @@ void osq_unlock(struct optimistic_spin_queue *lock)
>         node = this_cpu_ptr(&osq_node);
>         next = xchg(&node->next, NULL);
>         if (next) {
> -               WRITE_ONCE(next->locked, 1);
> +               smp_store_release(&next->locked, 1);
>                 return;
>         }
> 
>         next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
>         if (next)
> -               WRITE_ONCE(next->locked, 1);
> +               smp_store_release(&next->locked, 1);
>  }

The test passes with the above patch:

# dmesg|grep mx
[    7.010502] mx_test: osq_lock                    padding:  8 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds
[   12.014144] mx_test: osq_lock                    padding: 16 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
[   17.017572] mx_test: osq_lock                    padding: 24 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
[   22.019636] mx_test: osq_lock                    padding: 32 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
[   27.022192] mx_test: osq_lock                    padding: 40 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds
[   32.024907] mx_test: osq_lock                    padding: 48 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.004 seconds
[   37.026035] mx_test: osq_lock                    padding: 56 result: SUCCESS sum: 0 elements: 1000 elapsed: 5.000 seconds

I'll use David's advise about including the osq_lock code in my test,
so I can test it as a module, which will be more thorough.

If you submit this patch, feel free to add:

Tested-by: Håkon Bugge <haakon.bugge@oracle.com>


Thxs, Håkon