Re: [PATCH] futex: Avoid hash-bucket locking for mismatched waits

[Usama Arif]

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