Re: [PATCH 07/11] kasan: add compiler barriers to KUNIT_EXPECT_KASAN_FAIL

[Andrey Konovalov]

On Tue, Jan 12, 2021 at 9:18 AM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> On Tue, Jan 5, 2021 at 7:28 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> >
> > It might not be obvious to the compiler that the expression must be
> > executed between writing and reading to fail_data. In this case, the
> > compiler might reorder or optimize away some of the accesses, and
> > the tests will fail.
>
> Have you seen this happen in practice?

Yes.

> Are these accesses to fail_data that are optimized (in which case we
> could make it volatile)?

Yes. AFAIU compiler doesn't expect expression to change fail_data
fields, no those accesses and checks are optimized away.

> Note that compiler barriers won't probably help against removing
> memory accesses, they only prevent reordering.
>
> > +       barrier();                                              \
> >         expression;                                             \
> > +       barrier();                                              \
>
> The need for barriers is not obvious to the reader, so a comment in
> the code clarifying that would be nice.

Will add a comment in v2, thanks!