Commit 9270e1a7 authored by Alan Stern's avatar Alan Stern Committed by Paul E. McKenney
Browse files

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: default avatarAkira Yokosawa <akiyks@gmail.com>
Signed-off-by: default avatarAlan Stern <stern@rowland.harvard.edu>
Reviewed-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 3650b228

tools/memory-model/Documentation/litmus-tests.txt

+17 −0
Original line number Diff line number Diff line
@@ -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.