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

[Paul E. McKenney]

On Thu, Jul 27, 2023 at 06:10:12PM +0200, Jann Horn wrote:
> On Thu, Jul 27, 2023 at 5:44 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jul 27, 2023 at 03:57:47PM +0100, Will Deacon 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;
> > > > }
> >
> > This reads a little oddly, perhaps because it's a fragment from a larger
> > piece of code.
> 
> Yes, exactly. The READ_ONCE() would be in anon_vma_prepare(), which is
> a helper used to ensure that a VMA is associated with an anon_vma, and
> then the vma->anon_vma is used further down inside the fault handling
> path. Something like:
> 
> do_cow_fault
>   anon_vma_prepare
>     READ_ONCE(vma->anon_vma)
>     barrier()
>   finish_fault
>     do_set_pte
>       page_add_new_anon_rmap
>         folio_add_new_anon_rmap
>           __page_set_anon_rmap
>             [reads vma->anon_vma]
> 
> Anyway, I guess I'll follow what Paul and Matthew said and go with
> smp_load_acquire().

I thank you now, and you will thank youself later.  ;-)

							Thanx, Paul