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.
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