Re: [PATCH v11 RESEND 6/9] arm64: futex: support futex with FEAT_LSUI

Yeoreum Yun
2026-01-21T14:17:35Z

Hi Will,

> On Mon, Jan 19, 2026 at 10:17:47PM +0000, Yeoreum Yun wrote:
> > > > +"2:\n"
> > > > + _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0)
> > > > + : "+r" (ret), "+Q" (*uaddr), "+r" (*oldval)
> > > > + : "r" (newval)
> > > > + : "memory");
> > >
> > > Don't you need to update *oldval here if the CAS didn't fault?
> >
> > No. if CAS doesn't make fault the oldval update already.
>
> Sorry, it was the "+r" constraint with a pointer dereference that threw
> me but you have the "memory" clobber so it looks like this will work.
>
> > > > +
> > > > + for (i = 0; i < FUTEX_MAX_LOOPS; i++) {
> > > > + if (get_user(oval64.raw, uaddr64))
> > > > + return -EFAULT;
> > >
> > > Since oldval is passed to us as an argument, can we get away with a
> > > 32-bit get_user() here?
> >
> > It's not a probelm. but is there any sigificant difference?
>
> I think the code would be clearer if you only read what you actually
> use.
>
> > > > + nval64.raw = oval64.raw;
> > > > +
> > > > + if (futex_on_lo) {
> > > > + oval64.lo_futex.val = oldval;
> > > > + nval64.lo_futex.val = newval;
> > > > + } else {
> > > > + oval64.hi_futex.val = oldval;
> > > > + nval64.hi_futex.val = newval;
> > > > + }
> > > > +
> > > > + orig64.raw = oval64.raw;
> > > > +
> > > > + if (__lsui_cmpxchg64(uaddr64, &oval64.raw, nval64.raw))
> > > > + return -EFAULT;
> > > > +
> > > > + if (futex_on_lo) {
> > > > + oldval = oval64.lo_futex.val;
> > > > + other = oval64.lo_futex.other;
> > > > + orig_other = orig64.lo_futex.other;
> > > > + } else {
> > > > + oldval = oval64.hi_futex.val;
> > > > + other = oval64.hi_futex.other;
> > > > + orig_other = orig64.hi_futex.other;
> > > > + }
> > > > +
> > > > + if (other == orig_other) {
> > > > + ret = 0;
> > > > + break;
> > > > + }
> > > > + }
> > > > +
> > > > + if (!ret)
> > > > + *oval = oldval;
> > >
> > > Shouldn't we set *oval to the value we got back from the CAS?
> >
> > Since it's a "success" case, the CAS return and oldval must be the same.
> > That's why it doesn't matter to use got back from the CAS.
> > Otherwise, it returns error and *oval doesn't matter for
> > futex_atomic_cmpxchg_inatomic().
>
> Got it, but then the caller you have is very weird because e.g.
> __lsui_futex_atomic_eor() goes and does another get_user() on the next
> iteration instead of using the value returned by the CAS.
>
> It would probably be clearer if you restructured your CAS helper to look
> more like try_cmpxchg() and then the loop around it would be minimal.
> You might need to distinguish the faulting case from the comparison
> failure case with e.g. -EFAULT vs -EAGAIN.

Oh, thanks for pointing this out.I understand your point clearly now.
Yes, I’ll respin the patch accordingly. Thanks again!

--
Sincerely,
Yeoreum Yun