On 20/08/2026 17:23, Thomas Gleixner wrote:
> Usama!
>
> On Thu, Aug 20 2026 at 16:19, Usama Arif wrote:
>> On 14/08/2026 17:02, Thomas Gleixner wrote:
>>> --- a/kernel/futex/waitwake.c
>>> +++ b/kernel/futex/waitwake.c
>>> @@ -857,7 +857,21 @@ int futex_wait_setup(u32 __user *uaddr,
>>> CLASS(hbr, hbr)(&q->key);
>>> auto hb = hbr.hb;
>>>
>>> - futex_q_lock(q, hb);
>>> + futex_hb_waiters_inc(hb);
>>> + q->lock_ptr = &hb->lock;
>>> +
>>> + if (!spin_trylock(&hb->lock)) {
>>> + ret = get_user_inline(uval, uaddr);
>>> + if (ret) {
>>> + futex_hb_waiters_dec(hb);
>>> + return ret;
>>> + }
>>> + if (uval != val) {
>>> + futex_hb_waiters_dec(hb);
>>> + return -EWOULDBLOCK;
>>> + }
>>> + spin_lock(&hb->lock);
>>> + }
>>>
>>> ret = futex_get_value_locked(&uval, uaddr);
>>>
>> I like this approach; it makes the trade-off much better than my patch.
>> When spin_trylock() succeeds, the matching path performs only the existing
>> single user access, rather than the two accesses in my version.
>
> Correct.
>
>> Is it alright if I send v2 of the patch with your diff above? Or would you
>> like to send it?
>
> Go wild.
>
Thanks! I am going to try and run this in production workloads to gather data.
Should be done in the next few days and send it early next week.
> It lacks an __acquire(q->lock_ptr) annotation and a comment for this
> trylock muck and the open coded hb_waiter_inc(). The latter can just
> refer to futex_q_lock().
>
> Also the exit condition should be simplified into one "if (ret || uval !=
> val)", which removes four lines of code.
>
>> Please let me know how you would like it attributed if I send it.
>
> Suggested-by is fine.
>
> Thanks,
>
> tglx
