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SubjectRe: [RFC][PATCH v3]: documentation,atomic: Add new documents
Hi Will,

On Wed, Aug 02, 2017 at 10:45:32AM +0100, Will Deacon wrote:
[...]
>
> It's worth noting that we don't have the problem with any value-returning
> atomics, so all flavours of xchg in this test would be forbidden on arm64
> too.
>
> > C C-WillDeacon-MP+o-r+ai-rmb-o.litmus
> >
> > (*
> > * Expected result: Never.
> > *
> > * Desired litmus test, with atomic_inc() emulated by xchg_relaxed():
> > *
> > * WRITE_ONCE(x, 1); atomic_inc(&y);
> > * r0 = xchg_release(&y, 5); smp_rmb();
> > * r1 = READ_ONCE(x);
> > *
> > *
> > * WARN_ON(r0 == 0 && r1 == 0);
> > *)
> >
> > {
> > }
> >
> > P0(int *x, int *y)
> > {
> > WRITE_ONCE(*x, 1);
> > r0 = xchg_release(y, 5);
> > }
> >
> > P1(int *x, int *y)
> > {
> > r2 = xchg_relaxed(y, 1);
> > smp_rmb();
> > r1 = READ_ONCE(*x);
> > }
> >
> > exists
> > (0:r0=0 /\ 1:r1=0)
> >

How about a litmus test like this?

C C-AMO-global-transitivity.litmus

{
}

P0(int *x, int *y)
{
WRITE_ONCE(*x, 1);
r0 = xchg_release(y, 5);
}

P1(int *y, int *z)
{
atomic_inc(y);
smp_mb();
r1 = READ_ONCE(*z);
}

P2(int *x, int *z)
{
WRITE_ONCE(*z, 1);
smp_mb();
r2 = READ_ONCE(*x);
}

exists
(0:r0=0 /\ 1:r1=0 /\ 2:r2=0 )

Should we forbid the outcome in the exists-clause? I ask because I want
to know whether we can just treat atomic_inc() as a store, because if I
replace atomic_inc() with a WRITE(*y, 6), IIUC, the current model says
this could happen.

Thoughts?

Regards,
Boqun
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