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From:   Jann Horn <jannh@google.com>
Date:   Thu, 27 Jul 2023 18:10:12 +0200
Message-ID: <CAG48ez3OXbiruoATeSp-PZ9ZdDcFuwJ4+XCS6EgY_jrtcqqGcw@mail.gmail.com>
Subject: Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix
 anon_vma memory ordering
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Will Deacon <will@kernel.org>, paulmck@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>
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On Thu, Jul 27, 2023 at 5:44=E2=80=AFPM Alan Stern <stern@rowland.harvard.e=
du> 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) !=3D 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 =3D vma->anon_vma;
> > >
> > >   // this needs to be address-dependency-ordered against one of
> > >   // the loads from vma->anon_vma
> > >   struct anon_vma *root =3D 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().