On Mon, 2008-04-21 at 15:00 +0300, Avi Kivity wrote: > Linus Torvalds wrote: > > Finally, I don't think that comment is correct in the first place. It's > > not that simple. The thing is, even *with* the memory barrier in place, we > > may have: > > > > CPU#1 CPU#2 > > ===== ===== > > > > fast_gup: > > - read low word > > > > native_set_pte_present: > > - set low word to 0 > > - set high word to new value > > > > - read high word > > > > - set low word to new value > > > > and so you read a low word that is associated with a *different* high > > word! Notice? > > > > So trivial memory ordering is _not_ enough. > > > > So I think the code literally needs to be something like this > > > > #ifdef CONFIG_X86_PAE > > > > static inline pte_t native_get_pte(pte_t *ptep) > > { > > pte_t pte; > > > > retry: > > pte.pte_low = ptep->pte_low; > > smp_rmb(); > > pte.pte_high = ptep->pte_high; > > smp_rmb(); > > if (unlikely(pte.pte_low != ptep->pte_low) > > goto retry; > > return pte; > > } > > > > > > I think this is still broken. Suppose that after reading pte_high > native_set_pte() is called again on another cpu, changing pte_low back > to the original value (but with a different pte_high). You now have > pte_low from second native_set_pte() but pte_high from the first > native_set_pte(). I think the idea was that for user pages we only use set_pte_present() which does the low=0 thing first. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html