On Thu, Jul 27, 2023 at 04:07:32PM +0100, Matthew Wilcox wrote:
> On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote:
> > Assume that we are holding some kind of lock that ensures that the
> > only possible concurrent update to "vma->anon_vma" is that it changes
> > from a NULL pointer to a non-NULL pointer (using smp_store_release()).
> >
> >
> > if (READ_ONCE(vma->anon_vma) != NULL) {
> > // we now know that vma->anon_vma cannot change anymore
> >
> > // access the same memory location again with a plain load
> > struct anon_vma *a = vma->anon_vma;
> >
> > // this needs to be address-dependency-ordered against one of
> > // the loads from vma->anon_vma
> > struct anon_vma *root = a->root;
> > }
> >
> >
> > Is this fine? If it is not fine just because the compiler might
> > reorder the plain load of vma->anon_vma before the READ_ONCE() load,
> > would it be fine after adding a barrier() directly after the
> > READ_ONCE()?
> >
> > I initially suggested using READ_ONCE() for this, and then Linus and
> > me tried to reason it out and Linus suggested (if I understood him
> > correctly) that you could make the ugly argument that this works
> > because loads from the same location will not be reordered by the
> > hardware. So on anything other than alpha, we'd still have the
> > required address-dependency ordering because that happens for all
> > loads, even plain loads, while on alpha, the READ_ONCE() includes a
> > memory barrier. But that argument is weirdly reliant on
> > architecture-specific implementation details.
> >
> > The other option is to replace the READ_ONCE() with a
> > smp_load_acquire(), at which point it becomes a lot simpler to show
> > that the code is correct.
>
> Aren't we straining at gnats here? The context of this is handling a
> page fault, and we used to take an entire rwsem for read. I'm having
> a hard time caring about "the extra expense" of an unnecessarily broad
> barrier.
>
> Cost of an L3 cacheline miss is in the thousands of cycles. Cost of a
> barrier is ... tens?
Couldn't agree more!
Thanx, Paul
