Re: [RFC 0/3] tools/memory-model: Add litmus tests for atomic APIs

[Boqun Feng]

On Sat, Feb 15, 2020 at 07:25:50AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 14, 2020 at 10:27:44AM -0500, Alan Stern wrote:
> > On Fri, 14 Feb 2020, Boqun Feng wrote:
> > 
> > > A recent discussion raises up the requirement for having test cases for
> > > atomic APIs:
> > > 
> > > 	https://lore.kernel.org/lkml/20200213085849.GL14897@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > > 
> > > , and since we already have a way to generate a test module from a
> > > litmus test with klitmus[1]. It makes sense that we add more litmus
> > > tests for atomic APIs into memory-model.
> > 
> > It might be worth discussing this point a little more fully.  The 
> > set of tests in tools/memory-model/litmus-tests/ is deliberately rather 
> > limited.  Paul has a vastly more expansive set of litmus tests in a 
> > GitHub repository, and I am doubtful about how many new tests we want 
> > to keep in the kernel source.
> 
> Indeed, the current view is that the litmus tests in the kernel source
> tree are intended to provide examples of C-litmus-test-language features
> and functions, as opposed to exercising the full cross-product of
> Linux-kernel synchronization primitives.
> 
> For a semi-reasonable subset of that cross-product, as Alan says, please
> see https://github.com/paulmckrcu/litmus.
> 
> For a list of the Linux-kernel synchronization primitives currently
> supported by LKMM, please see tools/memory-model/linux-kernel.def.
> 

So how about I put those atomic API tests into a separate directory, say
Documentation/atomic/ ?

The problem I want to solve here is that people (usually who implements
the atomic APIs for new archs) may want some examples, which can help
them understand the API requirements and test the implementation. And
litmus tests are the perfect tool here (given that them can be
translated to test modules with klitmus). And I personally really think
this is something the LKMM group should maintain, that's why I put them
in the tools/memory-model/litmus-tests/. But I'm OK if we end up
deciding those should be put outside that directory.

Regards,
Boqun

> > Perhaps it makes sense to have tests corresponding to all the examples
> > in Documentation/, perhaps not.  How do people feel about this?
> 
> Agreed, we don't want to say that the set of litmus tests in the kernel
> source tree is limited for all time to the set currently present, but
> rather that the justification for adding more would involve useful and
> educational examples of litmus-test features and techniques rather than
> being a full-up LKMM test suite.
> 
> I would guess that there are litmus-test tricks that could usefully
> be added to tools/memory-model/litmus-tests.  Any nomination?  Perhaps
> handling CAS loops while maintaining finite state space?  Something else?
> 
> 							Thanx, Paul