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

Kirill A. Shutemov
2017-12-14T00:33:21Z

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.

--
Kirill A. Shutemov