On Tue, 20 Feb 2018, Paul E. McKenney wrote:
> From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>
> This commit adds a litmus test in which P0() and P1() form a lock-based S
> litmus test, with the addition of P2(), which observes P0()'s and P1()'s
Why do you call this an "S" litmus test? Isn't ISA2 a better
description?
> accesses with a full memory barrier but without the lock. This litmus
> test asks whether writes carried out by two different processes under the
> same lock will be seen in order by a third process not holding that lock.
> The answer to this question is "yes" for all architectures supporting
> the Linux kernel, but is "no" according to the current version of LKMM.
>
> A patch to LKMM is under development.
>
> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
> .../ISA2+pooncelock+pooncelock+pombonce.litmus | 41 ++++++++++++++++++++++
> 1 file changed, 41 insertions(+)
> create mode 100644 tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
Aren't these tests supposed to be described in litmus-tests/README?
> diff --git a/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
> new file mode 100644
> index 000000000000..7a39a0aaa976
> --- /dev/null
> +++ b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
> @@ -0,0 +1,41 @@
> +C ISA2+pooncelock+pooncelock+pombonce.litmus
> +
> +(*
> + * Result: Sometimes
> + *
> + * This test shows that the ordering provided by a lock-protected S
> + * litmus test (P0() and P1()) are not visible to external process P2().
> + * This is likely to change soon.
That last line may be premature. We haven't reached any consensus on
how RISC-V will handle this. If RISC-V allows the test then the memory
model can't forbid it.
Alan
> + *)
> +
> +{}
> +
> +P0(int *x, int *y, spinlock_t *mylock)
> +{
> + spin_lock(mylock);
> + WRITE_ONCE(*x, 1);
> + WRITE_ONCE(*y, 1);
> + spin_unlock(mylock);
> +}
> +
> +P1(int *y, int *z, spinlock_t *mylock)
> +{
> + int r0;
> +
> + spin_lock(mylock);
> + r0 = READ_ONCE(*y);
> + WRITE_ONCE(*z, 1);
> + spin_unlock(mylock);
> +}
> +
> +P2(int *x, int *z)
> +{
> + int r1;
> + int r2;
> +
> + r2 = READ_ONCE(*z);
> + smp_mb();
> + r1 = READ_ONCE(*x);
> +}
> +
> +exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
>
