Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Alan Stern
2018-07-10T15:34:51Z

On Tue, 10 Jul 2018, Andrea Parri wrote:

> > > ACQUIRE operations include LOCK operations and both smp_load_acquire()
> > > and smp_cond_acquire() operations. [BTW, the latter was replaced by
> > > smp_cond_load_acquire() in 1f03e8d2919270 ...]
> > >
> > > RELEASE operations include UNLOCK operations and smp_store_release()
> > > operations. [...]
> > >
> > > [...] after an ACQUIRE on a given variable, all memory accesses
> > > preceding any prior RELEASE on that same variable are guaranteed
> > > to be visible.
> >
> > As far as I can see, these statements remain valid.
>
> Interesting; ;-) What does these statement tells you ;-) when applied
> to a: and b: below?
>
> a: WRITE_ONCE(x, 1); // "preceding any prior RELEASE..."
> smp_store_release(&s, 1);
> smp_load_acquire(&s);
> b: WRITE_ONCE(y, 1); // "after an ACQUIRE..."

The first statement tells me that b: follows an ACQUIRE.

The second tells me that a: precedes a RELEASE.

And the third tells me that any READ_ONCE(x) statements coming po-after
b: would see x = 1 or a later value of x. (Of course, they would have
to see that anyway because of the cache coherency rules.)

More to the point, given:

P0()
{
WRITE_ONCE(x, 1);
a: smp_store_release(&s, 1);
}

P1()
{
b: r1 = smp_load_acquire(&s);
r2 = READ_ONCE(x);
}

the third statement tells me that if r1 = 1 (that is, if a: is prior to
b:) then r2 must be 1.

Alan