Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

["André Almeida" <andrealmeid@igalia.com>]

Em 12/03/2026 11:12, Florian Weimer escreveu:
> * Mathieu Desnoyers:
> 
>> On 2026-03-12 04:49, Florian Weimer wrote:
>>> * Mathieu Desnoyers:
>>>
>>>> + * This vDSO unlocks the robust futex by exchanging the content of
>>>> + * *uaddr with 0 with a store-release semantic. If the futex has
>>>> + * waiters, it sets bit 1 of *op_pending_addr, else it clears
>>>> + * *op_pending_addr. Those operations are within a code region
>>>> + * known by the kernel, making them safe with respect to asynchronous
>>>> + * program termination either from thread context or from a nested
>>>> + * signal handler.
>>>> + *
>>>> + * Expected use of this vDSO:
>>>> + *
>>>> + * if ((__vdso_robust_futex_unlock((u32 *) &mutex->__data.__lock, &pd->robust_head.list_op_pending)
>>>> + *     & FUTEX_WAITERS) != 0)
>>>> + *         futex_wake((u32 *) &mutex->__data.__lock, 1, private);
>>>> + * WRITE_ONCE(pd->robust_head.list_op_pending, 0);
>>> The comment could perhaps say that pd->robust_head is the
>>> thread-specific robust list that has been registered with
>>> set_robust_list.
> 
>> Good point. Considering that "robust_head" is the thread-specific
>> robust list registered with set_robust_list, I wonder if passing
>> &robust_head->list_op_pending is the right ABI choice there,
>> or if we should rather pass the robust_head pointer and offset it
>> within the vDSO.
> 
> I think set_robust_list has pointer and size arguments, so we should
> pass those two at least.
> 

The size argument for set_robust_list() has never been useful it seems, 
it just checks if (size == sizeof(*head)). I believe it was added in 
case the struct would ever be expanded, but that never happened and with 
set_robust_list2() in the horizon this is even less likely to ever happen.