Re: [PATCH locking/Documentation 1/2] Add note of release-acquire store vulnerability

Will Deacon
2016-09-30T09:20:15Z

On Fri, Sep 30, 2016 at 01:53:52PM +0800, Boqun Feng wrote:
> On Thu, Sep 29, 2016 at 10:23:22AM -0700, Paul E. McKenney wrote:
> > If two processes are related by a RELEASE+ACQUIRE pair, ordering can be
> > broken if a third process overwrites the value written by the RELEASE
> > operation before the ACQUIRE operation has a chance of reading it, for
> > example:
> >
> > P0(int *x, int *y)
> > {
> > WRITE_ONCE(*x, 1);
> > smp_wmb();
> ^^^^^^^^^^^
>
> What is this smp_wmb() for?
>
> > smp_store_release(y, 1);
> > }
> >
> > P1(int *y)
> > {
> > WRITE_ONCE(*y, 2);
>
> If we change this WRITE_ONCE to a relaxed atomic operation(e.g.
> xchg_relaxed(y, 2)), both herd and ppcmem said the exist-clause "y = 2
> /\ 2:r1 = 2 /\ 2:r2 = 0" wouldn't be triggered on PPC.
>
> I guess we will get the same behavior on ARM/ARM64, Will?
>
> If a normal store could break chain, while a RmW atomic won't, do we
> want to call it out in the document and build our memory model around
> this?

I think this is required to work by C11's definition of release sequences,
so any architecture that claims to support those with the same instructions
will need this to be forbidden.

Personally, I think that's a bug in C11, because I think it goes too far
in forbidding some hardware optimisations around relaxed xchg, but it is
what it is.

Will