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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 12 Jan 2021 20:57:03 +0100
Message-ID: <CAAeHK+yf4omWg-UHLiy4-6NjWXQs7pe4GyOgQOZnnpkhco1DGw@mail.gmail.com>
Subject: Re: [PATCH 07/11] kasan: add compiler barriers to KUNIT_EXPECT_KASAN_FAIL
To:     Alexander Potapenko <glider@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jan 12, 2021 at 8:50 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> On Tue, Jan 12, 2021 at 9:18 AM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Tue, Jan 5, 2021 at 7:28 PM Andrey Konovalov <andreyknvl@google.com> 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.