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From:   Alexander Potapenko <glider@google.com>
Date:   Tue, 12 Jan 2021 09:18:24 +0100
Message-ID: <CAG_fn=Uqp6dt5VGF8Dt6FeQzDgcEbVY8fs+5+wyMp2d1Z98sEw@mail.gmail.com>
Subject: Re: [PATCH 07/11] kasan: add compiler barriers to KUNIT_EXPECT_KASAN_FAIL
To:     Andrey Konovalov <andreyknvl@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 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?
Are these accesses to fail_data that are optimized (in which case we
could make it volatile), or some part of the expression?
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.