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

[Andrey Konovalov]

On Tue, Jan 12, 2021 at 8:50 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
>
> 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.

Ah, actually no, it reorders the expression and puts it after
fail_data fields checks. That's why I put the barriers.

> > Note that compiler barriers won't probably help against removing
> > memory accesses, they only prevent reordering.

But using WRITE/READ_ONCE() might also be a good idea, as technically
the compiler can optimize away the accesses.