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 <st...@rowland.harvard.edu>
AuthorDate: Sat, 03 Oct 2020 21:40:22 -04:00
Committer: Paul E. McKenney <paul...@kernel.org>
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 <aki...@gmail.com>
Signed-off-by: Alan Stern <st...@rowland.harvard.edu>
Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
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.
