Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix anon_vma memory ordering

[Jann Horn]

On Thu, Jul 27, 2023 at 5:07 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote:
> > 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?

Yeah, fair point. If it's hard to show correctness with READ_ONCE() we
can just use smp_load_acquire() and call it a day.