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

[Will Deacon]

On Thu, Jul 27, 2023 at 12:34:44PM -0400, Joel Fernandes wrote:
> > On Jul 27, 2023, at 10:57 AM, Will Deacon <will@xxxxxxxxxx> wrote:
> > On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote:
> >> 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'm _very_ wary of mixing READ_ONCE() and plain loads to the same variable,
> > as I've run into cases where you have sequences such as:
> > 
> >    // Assume *ptr is initially 0 and somebody else writes it to 1
> >    // concurrently
> > 
> >    foo = *ptr;
> >    bar = READ_ONCE(*ptr);
> >    baz = *ptr;
> > 
> > and you can get foo == baz == 0 but bar == 1 because the compiler only
> > ends up reading from memory twice.
> > 
> > That was the root cause behind f069faba6887 ("arm64: mm: Use READ_ONCE
> > when dereferencing pointer to pte table"), which was very unpleasant to
> > debug.
> 
> Will, Unless I am missing something fundamental, this case is different though.
> This case does not care about fewer reads. As long as the first read is volatile, the subsequent loads (even plain)
> should work fine, no?
> I am not seeing how the compiler can screw that up, so please do enlighten :).

I guess the thing I'm worried about is if there is some previous read of
'vma->anon_vma' which didn't use READ_ONCE() and the compiler kept the
result around in a register. In that case, 'a' could be NULL, even if
the READ_ONCE(vma->anon_vma) returned non-NULL.

The crux of the issue is that the compiler can break read-after-read
ordering if you don't use READ_ONCE() consistently. Sadly, judging by
the other part of the thread from Nadav, it's fiddly to fix this without
wrecking the codegen.

Will