linux-kernel - Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression

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Date:   Tue, 26 May 2020 13:02:46 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement
 expression

On Tue, May 26, 2020 at 12:42:16PM +0200, Arnd Bergmann wrote:
> On Thu, May 21, 2020 at 10:21 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@...glegroups.com> wrote:
> >
> > On Thu, May 21, 2020 at 7:22 AM 'Marco Elver' via Clang Built Linux
> > <clang-built-linux@...glegroups.com> wrote:
> > >
> > > It appears that compilers have trouble with nested statement
> > > expressions. Therefore remove one level of statement expression nesting
> > > from the data_race() macro. This will help us avoid potential problems
> > > in future as its usage increases.
> > >
> > > Link: https://lkml.kernel.org/r/20200520221712.GA21166@zn.tnic
> > > Acked-by: Will Deacon <will@...nel.org>
> > > Signed-off-by: Marco Elver <elver@...gle.com>
> >
> > Thanks Marco, I can confirm this series fixes the significant build
> > time regressions.
> >
> > Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >
> > More measurements in: https://github.com/ClangBuiltLinux/linux/issues/1032
> >
> > Might want:
> > Reported-by: Borislav Petkov <bp@...e.de>
> > Reported-by: Nathan Chancellor <natechancellor@...il.com>
> > too.
> 
> I find this patch only solves half the problem: it's much faster than
> without the
> patch, but still much slower than the current mainline version. As far as I'm
> concerned, I think the build speed regression compared to mainline is not yet
> acceptable, and we should try harder.
> 
> I have not looked too deeply at it yet, but this is what I found from looking
> at a file in a randconfig build:
> 
> Configuration: see https://pastebin.com/raw/R9erCwNj

So this .config actually has KCSAN enabled. Do you still see the slowdown
with that disabled? Although not ideal, having a longer compiler time when
the compiler is being asked to perform instrumentation doesn't seem like a
show-stopper to me.

Will