Re: [RFC PATCH] futex: Introduce __vdso_robust_futex_unlock

[Mathieu Desnoyers]

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.

If the process dies between those two instructions, your proposed
robust list code will be fooled and fall into the same bug that's
been lingering for 14 years.

Thanks,

Mathieu

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