Re: [RFC] tools/memory-model: Rule out OOTA

On Wed, Jul 23, 2025 at 09:26:32AM +0200, Hernan Ponce de Leon wrote:
> On 7/23/2025 2:43 AM, Paul E. McKenney wrote:
> > On Mon, Jan 06, 2025 at 10:40:03PM +0100, Jonas Oberhauser wrote:
> > > The current LKMM allows out-of-thin-air (OOTA), as evidenced in the following
> > > example shared on this list a few years ago:
> > 
> > Apologies for being slow, but I have finally added the litmus tests in
> > this email thread to the https://github.com/paulmckrcu/litmus repo.
> 
> I do not understand some of the comments in the preamble of the tests:
> 
> (*
>  * Result: Never
>  *
>  * But Sometimes in LKMM as of early 2025, given that 42 is a possible
>  * value for things like S19..
>  *
>  * https://lore.kernel.org/all/[email protected]/
>  *)
> 
> I see that herd7 reports one of the states to be [b]=S16. Is this supposed
> to be some kind of symbolic state (i.e., any value is possible)?

Exactly!

> The value in the "Result" is what we would like the model to say if we would
> have a perfect version of dependencies, right?

In this case, yes.

There are other cases elsewhere in which the "Result:" comment instead
records LKMM's current state, so that any deviation (whether right or
wrong) are noted.  Most recently, the 1800+ changes in luc/RelAcq.

> > It is quite likely that I have incorrectly intuited the missing portions
> > of the litmus tests, especially the two called out in the commit log
> > below.  If you have time, please do double-check.
> 
> I read the "On the other hand" from the commit log as "this fixes the
> problem". However I still get the following error when running C-JO-OOTA-7
> with herd7
> 
> Warning: File "manual/oota/C-JO-OOTA-7.litmus": Non-symbolic memory access
> found on '[0]' (User error)

Yes, my interpretation of the example in that URL didn't make any
sense at all to herd7.  So I would welcome a fix to this litmus test.
The only potential fixes that I found clearly went against the intent
of this litmus test.

My only real contribution in my coding of manual/oota/C-JO-OOTA-7.litmus
is showing how to initialize a local herd7 variable to contain a pointer
to a global variable.  ;-)

							Thanx, Paul

> Hernan>
> > And the updated (and condensed!) version of the C++ OOTA paper may be
> > found here, this time with a proposed change to the standard:
> > 
> > https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3692r1.pdf
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit fd17e8fceb75326e159ba3aa6fdb344f74f5c7a5
> > Author: Paul E. McKenney <[email protected]>
> > Date:   Tue Jul 22 17:21:19 2025 -0700
> > 
> >      manual/oota:  Add Jonas and Alan OOTA examples
> >      Each of these new litmus tests contains the URL of the email message
> >      that I took it from.
> >      Please note that I had to tweak the example leading up to
> >      C-JO-OOTA-4.litmus, and I might well have misinterpreted Jonas's "~"
> >      operator.
> >      Also, C-JO-OOTA-7.litmus includes a "*r2 = a" statement that makes herd7
> >      very unhappy.  On the other hand, initializing registers to the address
> >      of a variable is straight forward, as shown in the resulting litmus test.
> >      Signed-off-by: Paul E. McKenney <[email protected]>
> > 
> > diff --git a/manual/oota/C-AS-OOTA-1.litmus b/manual/oota/C-AS-OOTA-1.litmus
> > new file mode 100644
> > index 00000000..81a873a7
> > --- /dev/null
> > +++ b/manual/oota/C-AS-OOTA-1.litmus
> > @@ -0,0 +1,40 @@
> > +C C-AS-OOTA-1
> > +
> > +(*
> > + * Result: Sometimes
> > + *
> > + * Because smp_rmb() combined with smp_wmb() does not order earlier
> > + * reads against later writes.
> > + *
> > + * https://lore.kernel.org/all/[email protected]
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		*a = *b;
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		*b = *a;
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-AS-OOTA-2.litmus b/manual/oota/C-AS-OOTA-2.litmus
> > new file mode 100644
> > index 00000000..c672b0e7
> > --- /dev/null
> > +++ b/manual/oota/C-AS-OOTA-2.litmus
> > @@ -0,0 +1,33 @@
> > +C C-AS-OOTA-2
> > +
> > +(*
> > + * Result: Always
> > + *
> > + * If we were using C-language relaxed atomics instead of volatiles,
> > + * the compiler *could* eliminate the first WRITE_ONCE() in each process,
> > + * then also each process's local variable, thus having an undefined value
> > + * for each of those local variables.  But this cannot happen given that
> > + * we are using Linux-kernel _ONCE() primitives.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b)
> > +{
> > +	int r0 = READ_ONCE(*a);
> > +
> > +	WRITE_ONCE(*b, r0);
> > +	WRITE_ONCE(*b, 2);
> > +}
> > +
> > +P1(int *a, int *b)
> > +{
> > +	int r1 = READ_ONCE(*b);
> > +
> > +	WRITE_ONCE(*a, r0);
> > +	WRITE_ONCE(*a, 2);
> > +}
> > +
> > +exists ((0:r0=0 \/ 0:r0=2) /\ (1:r1=0 \/ 1:r1=2))
> > diff --git a/manual/oota/C-JO-OOTA-1.litmus b/manual/oota/C-JO-OOTA-1.litmus
> > new file mode 100644
> > index 00000000..6ab437b4
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-1.litmus
> > @@ -0,0 +1,40 @@
> > +C C-JO-OOTA-1
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But Sometimes in LKMM as of early 2025, given that 42 is a possible
> > + * value for things like S19..
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		*a = *b;
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		*b = *a;
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists (b=42)
> > diff --git a/manual/oota/C-JO-OOTA-2.litmus b/manual/oota/C-JO-OOTA-2.litmus
> > new file mode 100644
> > index 00000000..ad708c60
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-2.litmus
> > @@ -0,0 +1,44 @@
> > +C C-JO-OOTA-2
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But Sometimes in LKMM as of early 2025, given that 42 is a possible
> > + * value for things like S23.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2 = 0;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = *b;
> > +	}
> > +	WRITE_ONCE(*a, r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2 = 0;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = *a;
> > +	}
> > +	WRITE_ONCE(*b, r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists (b=42)
> > diff --git a/manual/oota/C-JO-OOTA-3.litmus b/manual/oota/C-JO-OOTA-3.litmus
> > new file mode 100644
> > index 00000000..633b8334
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-3.litmus
> > @@ -0,0 +1,46 @@
> > +C C-JO-OOTA-3
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But LKMM finds the all-ones result, perhaps due to not tracking
> > + * control dependencies out of the "if" statement.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	r2 = READ_ONCE(*b);
> > +	if (r1 == 1) {
> > +		r2 = *b;
> > +	}
> > +	WRITE_ONCE(*a, r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	r2 = READ_ONCE(*a);
> > +	if (r1 == 1) {
> > +		r2 = *a;
> > +	}
> > +	WRITE_ONCE(*b, r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-JO-OOTA-4.litmus b/manual/oota/C-JO-OOTA-4.litmus
> > new file mode 100644
> > index 00000000..cab7ebb6
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-4.litmus
> > @@ -0,0 +1,43 @@
> > +C C-JO-OOTA-4
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * And LKMM agrees, which might be a surprise.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +	int r3;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	r2 = *b;
> > +	r3 = r1 == 0;
> > +	WRITE_ONCE(*a, (r3 + 1) & r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +	int r3;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	r2 = *a;
> > +	r3 = r1 == 0;
> > +	WRITE_ONCE(*b, (r3 + 1) & r2);
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-JO-OOTA-5.litmus b/manual/oota/C-JO-OOTA-5.litmus
> > new file mode 100644
> > index 00000000..145c8378
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-5.litmus
> > @@ -0,0 +1,44 @@
> > +C C-JO-OOTA-5
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But LKMM finds the all-ones result, perhaps due r2 being unused.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*a);
> > +	}
> > +	*b = 1;
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*b);
> > +	}
> > +	*a = 1;
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +locations [0:r2;1:r2]
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-JO-OOTA-6.litmus b/manual/oota/C-JO-OOTA-6.litmus
> > new file mode 100644
> > index 00000000..942e6c82
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-6.litmus
> > @@ -0,0 +1,44 @@
> > +C C-JO-OOTA-6
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But LKMM finds the all-ones result, due to OOTA on r2.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*a);
> > +	}
> > +	*b = r2;
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*b);
> > +	}
> > +	*a = r2;
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +locations [0:r2;1:r2]
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-JO-OOTA-7.litmus b/manual/oota/C-JO-OOTA-7.litmus
> > new file mode 100644
> > index 00000000..31c0b8ae
> > --- /dev/null
> > +++ b/manual/oota/C-JO-OOTA-7.litmus
> > @@ -0,0 +1,47 @@
> > +C C-JO-OOTA-7
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * But LKMM finds the all-ones result, due to OOTA on r2.
> > + *
> > + * https://lore.kernel.org/all/[email protected]/
> > + *)
> > +
> > +{
> > +	0:r2=a;
> > +	1:r2=b;
> > +}
> > +
> > +P0(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*a);
> > +	}
> > +	*r2 = a;
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y)
> > +{
> > +	int r1;
> > +	int r2;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		r2 = READ_ONCE(*b);
> > +	}
> > +	*r2 = b;
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +locations [0:r2;1:r2]
> > +exists (0:r1=1 /\ 1:r1=1)
> > diff --git a/manual/oota/C-PM-OOTA-1.litmus b/manual/oota/C-PM-OOTA-1.litmus
> > new file mode 100644
> > index 00000000..e771e3c9
> > --- /dev/null
> > +++ b/manual/oota/C-PM-OOTA-1.litmus
> > @@ -0,0 +1,37 @@
> > +C C-PM-OOTA-1
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * LKMM agrees.
> > + *
> > + * https://lore.kernel.org/all/9a0dccbb-bfa7-4b33-ac1a-daa9841b609a@paulmck-laptop/
> > + *)
> > +
> > +{}
> > +
> > +P0(int *a, int *b, int *x, int *y) {
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*x);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		WRITE_ONCE(*a, *b);
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*y, 1);
> > +}
> > +
> > +P1(int *a, int *b, int *x, int *y) {
> > +	int r1;
> > +
> > +	r1 = READ_ONCE(*y);
> > +	smp_rmb();
> > +	if (r1 == 1) {
> > +		WRITE_ONCE(*b, *a);
> > +	}
> > +	smp_wmb();
> > +	WRITE_ONCE(*x, 1);
> > +}
> > +
> > +exists b=42
>