On Tue, Jun 14, 2022 at 03:48:11PM +0000, Paul Heidekrüger 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> > Cc: Marco Elver <elver@xxxxxxxxxx> > 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> > --- Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> thanks, - Joel > v2: > - Incorporate Alan Stern's feedback. > - Add suggested text by Alan Stern to clearly state how the branch and the > smp_mb() affect ordering. > - Add "Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>" based on the > above. > > .../Documentation/litmus-tests.txt | 37 ++++++++++++++----- > 1 file changed, 27 insertions(+), 10 deletions(-) > > diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt > index 8a9d5d2787f9..cc355999815c 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. > -- > 2.35.1 >
