Re: [PATCH v6] taskstats: fix data-race

[Dmitry Vyukov]

On Thu, Oct 24, 2019 at 4:40 PM Andrea Parri <parri.andrea@xxxxxxxxx> wrote:
>
> On Thu, Oct 24, 2019 at 03:58:40PM +0200, Dmitry Vyukov wrote:
> > On Thu, Oct 24, 2019 at 3:43 PM Andrea Parri <parri.andrea@xxxxxxxxx> wrote:
> > >
> > > > But why? I think kernel contains lots of such cases and it seems to be
> > > > officially documented by the LKMM:
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt
> > > > address dependencies and ppo
> > >
> > > Well, that same documentation also alerts about some of the pitfalls
> > > developers can incur while relying on dependencies.  I'm sure you're
> > > more than aware of some of the debate surrounding these issues.
> >
> > I thought that LKMM is finally supposed to stop all these
> > centi-threads around subtle details of ordering. And not we finally
> > have it. And it says that using address-dependencies is legal. And you
> > are one of the authors. And now you are arguing here that we better
> > not use it :) Can we have some black/white yes/no for code correctness
> > reflected in LKMM please :) If we are banning address dependencies,
> > don't we need to fix all of rcu uses?
>
> Current limitations of the LKMM are listed in tools/memory-model/README
> (and I myself discussed a number of them at LPC recently); the relevant
> point here seems to be:
>
> 1.      Compiler optimizations are not accurately modeled.  Of course,
>         the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
>         ability to optimize, but under some circumstances it is possible
>         for the compiler to undermine the memory model.  [...]
>
>         Note that this limitation in turn limits LKMM's ability to
>         accurately model address, control, and data dependencies.
>
> A less elegant, but hopefully more effective, way to phrase such point
> is maybe "feel free to rely on dependencies, but then do not blame the
> LKMM authors please".  ;-)

We are not going to blame LKMM authors :)

Acquire will introduce actual hardware barrier on arm/power/etc. Maybe
it does not matter here. But I feel if we start replacing all
load-depends/rcu with acquire, it will be noticeable overhead. So what
do we do in the context of the whole kernel?