Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

[Florian Weimer <fweimer@redhat.com>]

* 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.

Thanks,
Florian