[PATCH memory-model 4/8] tools/memory-model: Restrict to-r to read-read address dependency

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 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 3a4d3b49e85c..cfc1b8fd46da 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -81,7 +81,7 @@ 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. *)
-- 
2.40.0.rc2