Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

[Mathieu Desnoyers]

On 2026-03-12 15:10, Thomas Gleixner wrote:
> On Thu, Mar 12 2026 at 10:04, Mathieu Desnoyers wrote:
> > On 2026-03-12 09:46, André Almeida wrote:
> > > The interface that I would propose here would be a bit more "generic" or
> > > "flexible":
> > >
> > > __vdso_robust_futex_unlock(void *uaddr, int uval, struct
> > > robust_list_head *head, unsigned int flags)
> >
> > I agree on adding explicit "uval" and pointer to robust list head,
> > I'm not convinced that the rest is an improvement.
> >
> > This would require the caller to deal with errors, making it
> > more complex than a simple replacement for atomic xchg/cmpxchg.
> >
> > "flags" could be unsupported, so the handler would have to deal with
> > -EINVAL.
>
> What's the problem with that? pthread_mutex_unlock() has a return value
> too.

My aim is to use this vDSO as a replacement for atomic xchg and atomic
cmpxchg within library code. I am trying to make the transition as
straightforward as possible considering that this is a design bug
fix.

If adding error handling at that precise point of the libc robust mutex
unlock code is straightforward, I don't mind internally checking flags
and returning -EINVAL, but I'd want to hear about preference from the
libc people on this topic beforehand.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com