On Sat, Oct 03, 2020 at 09:40:22PM -0400, Alan Stern wrote: > 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> thanks, - Joel > > --- > > 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. >
