Re: Do we need to correct barriering in circular-buffers.rst?

Nick Desaulniers
2019-09-27T21:58:27Z

On Fri, Sep 27, 2019 at 1:43 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Sep 27, 2019 at 5:49 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > Barring LTO the above works for perf because of inter-translation-unit
> > function calls, which imply a compiler barrier.
> >
> > Now, when the compiler inlines, it looses that sync point (and thereby
> > subtlely changes semantics from the non-inline variant). I suspect LTO
> > does the same and can cause subtle breakage through this transformation.
>
> Do you have a bug report or godbolt link for the above? I trust that
> you're familiar enough with the issue to be able to quickly reproduce
> it? These descriptions of problems are difficult for me to picture in
> code or generated code, and when I try to read through
> memory-barriers.txt my eyes start to glaze over (then something else
> catches fire and I have to go put that out). Having a concise test
> case I think would better illustrate potential issues with LTO that
> we'd then be able to focus on trying to fix/support.
>

Further, if we identified a case were the existing invariants were
broken under LTO, such a case could be added to
CONFIG_DEBUG_LOCKING_API_SELFTESTS (or whatever the most appropriate
kself test is).

--
Thanks,
~Nick Desaulniers