Re: [RFC v2 3/3] tools/memory-model: litmus: Add two tests for unlock(A)+lock(B) ordering

[Paul E. McKenney]

On Tue, Oct 26, 2021 at 09:01:00AM +0200, Peter Zijlstra wrote:
> On Mon, Oct 25, 2021 at 10:54:16PM +0800, Boqun Feng wrote:
> > diff --git a/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> > new file mode 100644
> > index 000000000000..955b9c7cdc7f
> > --- /dev/null
> > +++ b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> > @@ -0,0 +1,33 @@
> > +C LB+unlocklockonceonce+poacquireonce
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * If two locked critical sections execute on the same CPU, all accesses
> > + * in the first must execute before any accesses in the second, even if
> > + * the critical sections are protected by different locks.
> 
> One small nit; the above "all accesses" reads as if:
> 
> 	spin_lock(s);
> 	WRITE_ONCE(*x, 1);
> 	spin_unlock(s);
> 	spin_lock(t);
> 	r1 = READ_ONCE(*y);
> 	spin_unlock(t);
> 
> would also work, except of course that's the one reorder allowed by TSO.

I applied this series with Peter's Acked-by, and with the above comment
reading as follows:

+(*
+ * Result: Never
+ *
+ * If two locked critical sections execute on the same CPU, all accesses
+ * in the first must execute before any accesses in the second, even if the
+ * critical sections are protected by different locks.  The one exception
+ * to this rule is that (consistent with TSO) a prior write can be reordered
+ * with a later read from the viewpoint of a process not holding both locks.
+ *)

Thank you all!

							Thanx, Paul