From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Akira Yokosawa <akiyks@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>, linux-arch@vger.kernel.org,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>
Subject: [PATCH RFC] tools/memory-model: Restrict to-r to read-read address dependency
Date:   Fri,  3 Feb 2023 20:19:13 +0000
Message-Id: <20230203201913.2555494-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

During a code-reading exercise of linux-kernel.cat CAT file, I generated
a graph to show the to-r relations. While likely not problematic for the
model, I found it confusing that a read-write address dependency would
show as a to-r edge on the graph.

This patch therefore restricts the to-r links derived from addr to only
read-read address dependencies, so that read-write address dependencies don't
show as to-r in the graphs. This should also prevent future users of to-r from
deriving incorrect relations. Note that a read-write address dep, obviously,
still ends up in the ppo relation via the to-w relation.

I verified that a read-read address dependency still shows up as a to-r
link in the graph, as it did before.

For reference, the problematic graph was generated with the following
command:
herd7 -conf linux-kernel.cfg \
   -doshow dep -doshow to-r -doshow to-w ./foo.litmus -show all -o OUT/

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 tools/memory-model/linux-kernel.cat | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index d70315fddef6..26e6f0968143 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -69,7 +69,7 @@ let dep = addr | data
 let rwdep = (dep | ctrl) ; [W]
 let overwrite = co | fr
 let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
-let to-r = addr | (dep ; [Marked] ; rfi)
+let to-r = (addr ; [R]) | (dep ; [Marked] ; rfi)
 let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
 
 (* Propagation: Ordering from release operations and strong fences. *)
-- 
2.39.1.519.gcb327c4b5f-goog