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Date:   Tue, 12 May 2020 21:07:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Will Deacon <will@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen
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On Tue, May 12, 2020 at 07:53:00PM +0200, Marco Elver wrote:
> I just ran a bunch of KCSAN tests. While this series alone would have
> passed the tests, there appears to be a problem with
> __READ_ONCE/__WRITE_ONCE. I think they should already be using
> 'data_race()', as otherwise we will get lots of false positives in
> future.
> 
> I noticed this when testing -tip/locking/kcsan, which breaks
> unfortunately, because I see a bunch of spurious data races with
> arch_atomic_{read,set} because "locking/atomics: Flip fallbacks and
> instrumentation" changed them to use __READ_ONCE()/__WRITE_ONCE().
> From what I see, the intent was to not double-instrument,
> unfortunately they are still double-instrumented because
> __READ_ONCE/__WRITE_ONCE doesn't hide the access from KCSAN (nor KASAN
> actually). I don't think we can use __no_sanitize_or_inline for the
> arch_ functions, because we really want them to be __always_inline
> (also to avoid calls to these functions in uaccess regions, which
> objtool would notice).
> 
> I think the easiest way to resolve this is to wrap the accesses in
> __*_ONCE with data_race().

But we can't... because I need arch_atomic_*() and __READ_ONCE() to not
call out to _ANYTHING_.

Sadly, because the compilers are 'broken' that whole __no_sanitize thing
didn't work, but I'll be moving a whole bunch of code into .c files with
all the sanitizers killed dead. And we'll be validating it'll not be
calling out to anything.

data_race() will include active calls to kcsan_{dis,en}able_current(),
and this must not happen.