linux-kernel - Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test

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Date:   Wed, 21 Feb 2018 09:53:21 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, parri.andrea@...il.com, will.deacon@....com,
        peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, nborisov@...e.com
Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S
 lock-based external-view litmus test

On Wed, Feb 21, 2018 at 11:50:31AM -0500, Alan Stern wrote:
> 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@...land.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.

No argument.

> > > > 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@...land.harvard.edu>
> > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.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.

I did, please accept my apologies and please see below.

> > > > 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.

Agreed.

> > 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.

Here is hoping!

							Thanx, Paul

------------------------------------------------------------------------

commit e6658d1d7fcc6391f3d00beaadc484243123a893
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Wed Feb 21 09:49:01 2018 -0800

    tools/memory-order: Add documentation of new litmus test
    
    The litmus-tests/README file lacked any mention of then litmus test
    named ISA2+pooncelock+pooncelock+pombonce.litmus.  This commit therefore
    adds this test.
    
    Reported-by: Alan Stern <stern@...land.harvard.edu>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/tools/memory-model/litmus-tests/README b/tools/memory-model/litmus-tests/README
index dca7d823ad57..aff3eb90e067 100644
--- a/tools/memory-model/litmus-tests/README
+++ b/tools/memory-model/litmus-tests/README
@@ -32,6 +32,11 @@ IRIW+poonceonces+OnceOnce.litmus
 	order of a pair of writes, where each write is to a different
 	variable by a different process?
 
+ISA2+pooncelock+pooncelock+pombonce.litmus
+	Tests whether the ordering provided by a lock-protected S litmus
+	test is visible to an external process whose accesses are
+	separated by smp_mb().
+
 ISA2+poonceonces.litmus
 	As below, but with store-release replaced with WRITE_ONCE()
 	and load-acquire replaced with READ_ONCE().