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    SubjectRe: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen
    On Wed, 13 May 2020 at 15:24, Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Wed, May 13, 2020 at 03:15:55PM +0200, Marco Elver wrote:
    > > So far so good, except: both __no_sanitize_or_inline and
    > > __no_kcsan_or_inline *do* avoid KCSAN instrumenting plain accesses, it
    > > just doesn't avoid explicit kcsan_check calls, like those in
    > > READ/WRITE_ONCE if KCSAN is enabled for the compilation unit. That's
    > > just because macros won't be redefined just for __no_sanitize
    > > functions. Similarly, READ_ONCE_NOCHECK does work as expected, and its
    > > access is unchecked.
    > >
    > > This will have the expected result:
    > > __no_sanitize_or_inline void foo(void) { x++; } // no data races reported
    > >
    > > This will not work as expected:
    > > __no_sanitize_or_inline void foo(void) { READ_ONCE(x); } // data
    > > races are reported
    > >
    > > All this could be fixed if GCC devs would finally take my patch to
    > > make -fsanitize=thread distinguish volatile [1], but then we have to
    > > wait ~years for the new compilers to reach us. So please don't hold
    > > your breath for this one any time soon.
    > > [1] https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544452.html
    >
    > Right, but that does not address the much larger issue of the attribute
    > vs inline tranwreck :/

    Could you check if Clang is equally broken for you? I think GCC and
    Clang have differing behaviour on this. No idea what it takes to fix
    GCC though.

    > Also, could not this compiler instrumentation live as a kernel specific
    > GCC-plugin instead of being part of GCC proper? Because in that case,
    > we'd have much better control over it.

    I'd like it if we could make it a GCC-plugin for GCC, but how? I don't
    see a way to affect TSAN instrumentation. FWIW Clang already has
    distinguish-volatile support (unreleased Clang 11).

    Thanks,
    -- Marco

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