linux-kernel - [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on RMWs in tools/...

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Date: Tue,  4 Jun 2024 18:04:40 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: paulmck@...nel.org
Cc: stern@...land.harvard.edu,
	parri.andrea@...il.com,
	will@...nel.org,
	peterz@...radead.org,
	boqun.feng@...il.com,
	npiggin@...il.com,
	dhowells@...hat.com,
	j.alglave@....ac.uk,
	luc.maranget@...ia.fr,
	akiyks@...il.com,
	dlustig@...dia.com,
	joel@...lfernandes.org,
	urezki@...il.com,
	quic_neeraju@...cinc.com,
	frederic@...nel.org,
	linux-kernel@...r.kernel.org,
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
	Viktor Vafeiadis <viktor@...-sws.org>
Subject: [PATCHv2 3/4] tools/memory-model: Define effect of Mb tags on RMWs in tools/...

Herd7 transforms successful RMW with Mb tags by inserting smp_mb() fences
around them. We emulate this by considering imaginary po-edges before the
RMW read and before the RMW write, and extending the smp_mb() ordering
rule, which currently only applies to real po edges that would be found
around a really inserted smp_mb(), also to cases of the only imagined po
edges.

Reported-by: Viktor Vafeiadis <viktor@...-sws.org>
Suggested-by: Alan Stern <stern@...land.harvard.edu>
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
---
 tools/memory-model/linux-kernel.cat | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index adf3c4f41229..d7e7bf13c831 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -34,6 +34,16 @@ let R4rmb = R \ Noreturn	(* Reads for which rmb works *)
 let rmb = [R4rmb] ; fencerel(Rmb) ; [R4rmb]
 let wmb = [W] ; fencerel(Wmb) ; [W]
 let mb = ([M] ; fencerel(Mb) ; [M]) |
+	(*
+	 * full-barrier RMWs (successful cmpxchg(), xchg(), etc.) act as
+	 * though there were enclosed by smp_mb().
+	 * The effect of these virtual smp_mb() is formalized by adding
+	 * Mb tags to the read and write of the operation, and providing
+	 * the same ordering as though there were additional po edges
+	 * between the Mb tag and the read resp. write.
+	 *)
+	([M] ; po ; [Mb & R]) |
+	([Mb & W] ; po ; [M]) |
 	([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
 	([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
 	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
-- 
2.34.1