Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Yu, Yu-cheng]

On 1/26/2021 1:40 AM, Peter Zijlstra wrote:
> On Tue, Jan 26, 2021 at 09:46:36AM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 25, 2021 at 07:27:09PM +0100, Borislav Petkov wrote:
> >
> > > > +		pte_t old_pte, new_pte;
> > > > +
> > > > +		do {
> > > > +			old_pte = READ_ONCE(*ptep);
> > > > +			new_pte = pte_wrprotect(old_pte);
> > >
> > > Maybe I'm missing something but those two can happen outside of the
> > > loop, no? Or is *ptep somehow changing concurrently while the loop is
> > > doing the CMPXCHG and you need to recreate it each time?
> > >
> > > IOW, you can generate upfront and do the empty loop...
> > >
> > > > +
> > > > +		} while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
> > > > +
> > > > +		return;
> > > > +	}
> >
> > Empty loop would be wrong, but that wants to be written like:
> >
> > 	old_pte = READ_ONCE(*ptep);
> > 	do {
> > 		new_pte = pte_wrprotect(old_pte);
> > 	} while (try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
>
> ! went missing, too early, moar wake-up juice.
>
> > Since try_cmpxchg() will update old_pte on failure.

Thanks Peter!  I will fix that.

--
Yu-cheng