Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

[André Almeida]

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.