[tip: core/rcu] tools: memory-model: Document that the LKMM can easily miss control dependencies

[tip-bot2 for Alan Stern]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     9270e1a744f8ed953009b0e94b26ed0912d9ec1c
Gitweb:        https://git.kernel.org/tip/9270e1a744f8ed953009b0e94b26ed0912d9ec1c
Author:        Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
AuthorDate:    Sat, 03 Oct 2020 21:40:22 -04:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 26 Oct 2020 16:18:53 -07:00

tools: memory-model: Document that the LKMM can easily miss control dependencies

Add a small section to the litmus-tests.txt documentation file for
the Linux Kernel Memory Model explaining that the memory model often
fails to recognize certain control dependencies.

Suggested-by: Akira Yokosawa <akiyks@xxxxxxxxx>
Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 tools/memory-model/Documentation/litmus-tests.txt | 17 ++++++++++++++-
 1 file changed, 17 insertions(+)

diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt
index 2f840dc..8a9d5d2 100644
--- a/tools/memory-model/Documentation/litmus-tests.txt
+++ b/tools/memory-model/Documentation/litmus-tests.txt
@@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory model (LKMM) include:
 	carrying a dependency, then the compiler can break that dependency
 	by substituting a constant of that value.
 
+	Conversely, LKMM sometimes doesn't recognize that a particular
+	optimization is not allowed, and as a result, thinks that a
+	dependency is not present (because the optimization would break it).
+	The memory model misses some pretty obvious control dependencies
+	because of this limitation.  A simple example is:
+
+		r1 = READ_ONCE(x);
+		if (r1 == 0)
+			smp_mb();
+		WRITE_ONCE(y, 1);
+
+	There is a control dependency from the READ_ONCE to the WRITE_ONCE,
+	even when r1 is nonzero, but LKMM doesn't realize this and thinks
+	that the write may execute before the read if r1 != 0.  (Yes, that
+	doesn't make sense if you think about it, but the memory model's
+	intelligence is limited.)
+
 2.	Multiple access sizes for a single variable are not supported,
 	and neither are misaligned or partially overlapping accesses.