Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 4 Jun 2018 10:17:47 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        <andrea.parri@amarulasolutions.com>,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nick Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Roman Pen <roman.penyaev@profitbricks.com>
Subject: Re: LKMM litmus test for Roman Penyaev's rcu-rr
In-Reply-To: <20180602144440.GI3593@linux.vnet.ibm.com>
Message-ID: <Pine.LNX.4.44L0.1806041014250.1380-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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