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    SubjectRe: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test
    On Tue, 20 Feb 2018, Paul E. McKenney wrote:

    > From: Alan Stern <stern@rowland.harvard.edu>
    >
    > 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@rowland.harvard.edu>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > ---
    > .../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)
    >


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