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

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Date:   Wed, 13 May 2020 15:24:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
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, 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 :/

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.