Re: [PATCHv4 09/12] x86/mm: Provide pmdp_establish() helper

Andrew Morton
2017-12-14T00:36:41Z

On Thu, 14 Dec 2017 03:33:18 +0300 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:

> On Wed, Dec 13, 2017 at 04:09:51PM -0800, Andrew Morton wrote:
> > > @@ -181,6 +182,40 @@ static inline pmd_t native_pmdp_get_and_clear(pmd_t *pmdp)
> > > #define native_pmdp_get_and_clear(xp) native_local_pmdp_get_and_clear(xp)
> > > #endif
> > >
> > > +#ifndef pmdp_establish
> > > +#define pmdp_establish pmdp_establish
> > > +static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> > > + unsigned long address, pmd_t *pmdp, pmd_t pmd)
> > > +{
> > > + pmd_t old;
> > > +
> > > + /*
> > > + * If pmd has present bit cleared we can get away without expensive
> > > + * cmpxchg64: we can update pmdp half-by-half without racing with
> > > + * anybody.
> > > + */
> > > + if (!(pmd_val(pmd) & _PAGE_PRESENT)) {
> > > + union split_pmd old, new, *ptr;
> > > +
> > > + ptr = (union split_pmd *)pmdp;
> > > +
> > > + new.pmd = pmd;
> > > +
> > > + /* xchg acts as a barrier before setting of the high bits */
> > > + old.pmd_low = xchg(&ptr->pmd_low, new.pmd_low);
> > > + old.pmd_high = ptr->pmd_high;
> > > + ptr->pmd_high = new.pmd_high;
> > > + return old.pmd;
> > > + }
> > > +
> > > + {
> > > + old = *pmdp;
> > > + } while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
> >
> > um, what happened here?
>
> Ouch.. Yeah, we need 'do' here. :-/
>
> Apparently, it's a valid C code that would run the body once and it worked for
> me because I didn't hit the race condition.

So how the heck do we test this? Add an artificial delay on the other
side to open the race window?