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

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Date:   Tue, 12 May 2020 20:55:35 +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>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

On Tue, 12 May 2020 at 19:53, Marco Elver <elver@...gle.com> wrote:
>
> On Tue, 12 May 2020 at 10:18, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Mon, May 11, 2020 at 09:41:32PM +0100, Will Deacon wrote:
> > > Hi folks,
> > >
> > > (trimmed CC list since v4 since this is largely just a rebase)
> > >
> > > This is version five of the READ_ONCE() codegen improvement series that
> > > I've previously posted here:
> > >
> > > RFC: https://lore.kernel.org/lkml/20200110165636.28035-1-will@kernel.org
> > > v2:  https://lore.kernel.org/lkml/20200123153341.19947-1-will@kernel.org
> > > v3:  https://lore.kernel.org/lkml/20200415165218.20251-1-will@kernel.org
> > > v4:  https://lore.kernel.org/lkml/20200421151537.19241-1-will@kernel.org
> > >
> > > The main change since v4 is that this is now based on top of the KCSAN
> > > changes queued in -tip (locking/kcsan) and therefore contains the patches
> > > necessary to avoid breaking sparc32 as well as some cleanups to
> > > consolidate {READ,WRITE}_ONCE() and data_race().
> > >
> > > Other changes include:
> > >
> > >   * Treat 'char' as distinct from 'signed char' and 'unsigned char' for
> > >     __builtin_types_compatible_p()
> > >
> > >   * Add a compile-time assertion that the argument to READ_ONCE_NOCHECK()
> > >     points at something the same size as 'unsigned long'
> > >
> > > I'm happy for all of this to go via -tip, or I can take it via arm64.
> >
> > Looks good to me; Thanks!
> >
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
> 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().

I just sent https://lkml.kernel.org/r/20200512183839.2373-1-elver@google.com
-- note that, using __*_ONCE in arch_atomic_{read,set} will once again
double-instrument with this. Overall there are 2 options:
1. provide __READ_ONCE/__WRITE_ONCE wrapped purely in data_race(), or
2. make __READ_ONCE/__WRITE_ONCE perform an atomic check so we may
still catch races with plain accesses.
The patch I sent does (2). It is inevitable that these will be used in
places that we did not expect, purely to get around the type check,
which is why I thought it might be the more conservative approach.

Thoughts?

Thanks,
-- Marco