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Subject[PATCH] 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@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

---

tools/memory-model/Documentation/litmus-tests.txt | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

Index: usb-devel/tools/memory-model/Documentation/litmus-tests.txt
===================================================================
--- usb-devel.orig/tools/memory-model/Documentation/litmus-tests.txt
+++ usb-devel/tools/memory-model/Documentation/litmus-tests.txt
@@ -946,6 +946,23 @@ Limitations of the Linux-kernel memory m
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.

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