Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

[Sebastian Andrzej Siewior]

On 2026-03-12 18:52:43 [-0400], Mathieu Desnoyers wrote:
> On 2026-03-12 18:23, Thomas Gleixner wrote:
> > On Wed, Mar 11 2026 at 14:54, Mathieu Desnoyers wrote:
> [...]
> > 
> > TBH, all of this is completely overengineered and tasteless bloat.
> > 
> > The exactly same thing can be achieved by doing the obvious:
> > 
> > struct robust_list_head2 {
> > 	struct robust_list_head		rhead;
> >          u32				unlock_val;
> > };
> > 
> > // User space
> > unlock(futex)
> > {
> >          struct robust_list_head2 *h = ....;
> > 
> >          h->unlock_val = 0;
> >          h->rhead.list_op_pending = .... | FUTEX_ROBUST_UNLOCK;
> > 
> >          xchg(futex->uval, h->unlock_val);
> 
> Here is the problem with your proposed approach:
> 
>   "XCHG — Exchange Register/Memory With Register"
>                                         ^^^^^^^^
> 
> So only one of the xchg arguments can be a memory location.
> Therefore, you will end up needing an extra store after xchg
> to store the content of the result register into h->unlock_val.

But can't we also assign a role to pthread_mutex_destroy() here? So it
would ensure that the futex death cleanup did run for every task having
access to this memory? So it is either 0 or pid-of-dead-task before this
memory location can be used again?

> Thanks,
> 
> Mathieu

Sebastian