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

Alan Stern
2018-07-11T14:19:49Z

On Wed, 11 Jul 2018, Andrea Parri wrote:

> > > Does something like "po; [UL]; rf; [LKR]; po" fit in with the rest
> > > of the model? If so, maybe that solves the asymmetry and also
> > > legalizes the approach of putting fence.tso in front?
> >
> > That would work just as well. For this version of the patch it
> > doesn't make any difference, because nothing that comes po-after the
> > LKR is able to directly read the value stored by the UL.
>
> Consider:
>
> C v2-versus-v3
>
> {}
>
> P0(spinlock_t *s, int *x)
> {
> spin_lock(s); /* A */
> spin_unlock(s);
> spin_lock(s);
> WRITE_ONCE(*x, 1); /* B */
> spin_unlock(s);
> }
>
> P1(spinlock_t *s, int *x)
> {
> int r0;
> int r1;
>
> r0 = READ_ONCE(*x); /* C */
> smp_rmb();
> r1 = spin_is_locked(s); /* D */
> }
>
> With v3, it's allowed that C reads from B and D reads from (the LKW of) A;
> this is not allowed with v2 (unless I mis-applied/mis-read v2).

Correct. But it doesn't affect the end result, because both versions
allow C to read from B while D reads from the second spin_lock(), and
there's no way to distinguish that from the case where D reads from A.

If we were talking about arbitrary integers and rmw-acquire updates,
there _would_ be a difference. But not with spinlocks.

Alan