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

[Matthew Wilcox]

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?