linux-kernel - Re: [PATCH v5 00/18] Rework READ

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Message-ID: <CANpmjNM5dD1VH0hoQwsZYEL=mhWunKwAEJMQgASzHSN019OCnw@mail.gmail.com>
Date:   Wed, 13 May 2020 15:58:30 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

On Wed, 13 May 2020 at 15:24, Peter Zijlstra <peterz@...radead.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