[PATCH memory-model 5/7] tools/memory-model: Define effect of Mb tags on RMWs in tools/...

Paul E. McKenney
2025-02-20T16:14:05Z

From: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>

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@mpi-sws.org>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Tested-by: Boqun Feng <boqun.feng@gmail.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 adf3c4f412296..d7e7bf13c831b 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]) |
--
2.40.1