Re: LKMM litmus test for Roman Penyaev's rcu-rr

[Alan Stern <stern@rowland.harvard.edu>]

On Sat, 2 Jun 2018, Paul E. McKenney wrote:

> One crude but effective workaround is to replicate the code following the
> "if" statement into both legs of the "if" statement.  This has the effect
> of extending the control dependency to cover all of the code that used to
> follow the "if" statement, leveraging herd's current limited knowledge of
> compiler optimization.  This workaround would of course be hopeless for
> general Linux-kernel code, but should be at least semi-acceptable for the
> very small snippets of code that can be accommodated within litmus tests.
> 
> Please see the litmus test shown below, which uses this workaround,
> allowing the smp_store_release() to be downgraded to WRITE_ONCE().
> 
> Given this workaround, crude though it might be, I believe that we can
> take a more measured approach to identifying a longer-term solution.
> 
> Thoughts?

Yes, this works, although it is clearly just a stopgap.  And obviously
it can't be applied in situations where one of the legs of the "if"  
statement contains a non-trivial branch.

In the long run, I don't think this problem is solvable.  At least, not 
for all cases.  It requires too much guesswork about what optimizations 
a compiler might do.

Alan