Re: [patch v2 07/11] futex: Add support for unlocking robust futexes

[André Almeida]

Em 26/03/2026 19:23, Thomas Gleixner escreveu:
> On Fri, Mar 20 2026 at 14:14, André Almeida wrote:
> > Em 19/03/2026 20:24, Thomas Gleixner escreveu:
> > > The sys_futex() case [ab]uses the @uaddr2 argument to hand the pointer to
> > > the kernel. This argument is only evaluated when the FUTEX_ROBUST_UNLOCK
> > > bit is set and is therefore backward compatible.
> > I didn't find anywhere in the commit message that says what this
> > pointers points to, so I would add:
> >
> > "@uaddr2 argument to hand the address of robust list pending op to the
> > kernel"
>
> Right.
>
> > and also explain why we can't use
> > current->futex.robust_list->list_op_pending (if I understood it
> > correctly why):
> >
> > "Instead of using the list_op_pending address found at
> > current->futex.robust_list, use the address explicitly set by the user
> > in the syscall arguments to avoid racing with set_robust_list()"
>
> No. The task can't be in the futex syscall and update the robust list
> pointer concurrently. :)

Oh, that's right...

> The reason is to avoid the lookup of the robust list pointer and
> retrieving the pending op pointer. User space has the pointer already
> there so it can just put it into the @uaddr2 argument. Aside of that
> this allows the usage of multiple robust lists in the future w/o any
> changes to the internal functions as they just operate on the supplied
> pointer. Only in the unresolvable fault case the kernel checks whether
> there is a matching robust list registered and clears it to avoid
> further trouble.

Ok, makes sense!

> I'll amend the change log.

Thanks for the clarification.

> Thanks,
>
>          tglx