> On 8. Jul 2022, at 20:47, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Fri, Jul 08, 2022 at 10:45:06AM -0400, Alan Stern wrote: >> On Fri, Jul 08, 2022 at 01:44:06PM +0200, Marco Elver wrote: >>> On Tue, 14 Jun 2022 at 17:49, Paul Heidekrüger >>> <paul.heidekrueger@xxxxxxxxx> wrote: >>>> As discussed, clarify LKMM not recognizing certain kinds of orderings. >>>> In particular, highlight the fact that LKMM might deliberately make >>>> weaker guarantees than compilers and architectures. >>>> >>>> Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@xxxxxxxxxxxxxxxxxxxxxxxx/T/#u >>>> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> >>>> Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> >>> >>> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> >>> >>> However with the Co-developed-by, this is missing Alan's SOB. >> >> For the record: >> >> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> >> >> (Note that according to Documentation/process/submitting-patches.rst, >> the submitting author's SOB is supposed to come last.) > > And this is what I ended up with. Please provide additional feedback > as needed, and in the meantime, thank you all! > > Thanx, Paul Looks great - my first commit in the Linux kernel! Thanks everyone! Paul > ------------------------------------------------------------------------ > > commit 3c7753e959706f39e1ee183ef8dcde3b4cfbb4c7 > Author: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> > Date: Tue Jun 14 15:48:11 2022 +0000 > > tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt > > As discussed, clarify LKMM not recognizing certain kinds of orderings. > In particular, highlight the fact that LKMM might deliberately make > weaker guarantees than compilers and architectures. > > Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@xxxxxxxxxxxxxxxxxxxxxxxx/T/#u > Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> > Reviewed-by: Marco Elver <elver@xxxxxxxxxx> > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Cc: Charalampos Mainas <charalampos.mainas@xxxxxxxxx> > Cc: Pramod Bhatotia <pramod.bhatotia@xxxxxxxxx> > Cc: Soham Chakraborty <s.s.chakraborty@xxxxxxxxxx> > Cc: Martin Fink <martin.fink@xxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt > index 8a9d5d2787f9e..cc355999815cb 100644 > --- a/tools/memory-model/Documentation/litmus-tests.txt > +++ b/tools/memory-model/Documentation/litmus-tests.txt > @@ -946,22 +946,39 @@ 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: > + Conversely, LKMM will sometimes overestimate the amount of > + reordering compilers and CPUs can carry out, leading it to miss > + some pretty obvious cases of ordering. 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.) > + The WRITE_ONCE() does not depend on the READ_ONCE(), and as a > + result, LKMM does not claim ordering. However, even though no > + dependency is present, the WRITE_ONCE() will not be executed before > + the READ_ONCE(). There are two reasons for this: > + > + The presence of the smp_mb() in one of the branches > + prevents the compiler from moving the WRITE_ONCE() > + up before the "if" statement, since the compiler has > + to assume that r1 will sometimes be 0 (but see the > + comment below); > + > + CPUs do not execute stores before po-earlier conditional > + branches, even in cases where the store occurs after the > + two arms of the branch have recombined. > + > + It is clear that it is not dangerous in the slightest for LKMM to > + make weaker guarantees than architectures. In fact, it is > + desirable, as it gives compilers room for making optimizations. > + For instance, suppose that a 0 value in r1 would trigger undefined > + behavior elsewhere. Then a clever compiler might deduce that r1 > + can never be 0 in the if condition. As a result, said clever > + compiler might deem it safe to optimize away the smp_mb(), > + eliminating the branch and any ordering an architecture would > + guarantee otherwise. > > 2. Multiple access sizes for a single variable are not supported, > and neither are misaligned or partially overlapping accesses.
