On Fri, Jul 28, 2023 at 01:35:43PM -0400, Joel Fernandes wrote:
> On Fri, Jul 28, 2023 at 8:44 AM Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > 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.
>
> If I can be a bit brave enough to say -- that appears to be a compiler
> bug to me. It seems that the compiler in such an instance violates the
> "Sequential Consistency Per Variable" rule? I mean if it can't even
> keep SCPV true for a same memory-location load (plain or not) for a
> sequence of code, how can it expect the hardware to.
It's not a compiler bug. In this example, some other thread performs a
write that changes vma->anon_vma from NULL to non-NULL. This write
races with the plain reads, and compilers are not required to obey the
"Sequential Consistency Per Variable" rule (or indeed, any rule) when
there is a data race.
Alan Stern
