On Sun, Oct 04, 2020 at 05:07:47PM -0400, j...@joelfernandes.org wrote:
> 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 <aki...@gmail.com>
> > Signed-off-by: Alan Stern <st...@rowland.harvard.edu>
>
> Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org>
Good addition! Applied, and thank you all!!!
Thanx, Paul
> 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.
> >
