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SubjectRe: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test
On Wed, 21 Feb 2018, Paul E. McKenney wrote:

> On Wed, Feb 21, 2018 at 10:09:00AM -0500, Alan Stern wrote:
> > 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?
>
> Indeed, the name of the test is in fact ISA2.

Sure; and the Changelog entry should reflect this.

> > > 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?

You apparently missed this recommendation.

> > > 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.
>
> Agreed. How about this? If the RISC-V question is answered by the
> end of next week, I update accordingly. If not, I update the comment
> to give the details.

The README also should be updated.

> Hey, at least having the memory model go in at about the same time as
> a new architecture is giving us good practice! ;-)

Hopefully things will settle down in a week or two.

Alan

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