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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 20 May 2020 09:32:43 -0700
Message-ID: <CAKwvOd=ga4cjJCQASVH=Vwjkk5_Qh7b_TtoTSBBoLiMENL8hOQ@mail.gmail.com>
Subject: Re: [rcu] 2f08469563: BUG:kernel_reboot-without-warning_in_boot_stage
To:     Marco Elver <elver@google.com>
Cc:     george.burgess.iv@gmail.com, Kan Liang <kan.liang@linux.intel.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        kernel test robot <rong.a.chen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, LKP <lkp@lists.01.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>
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On Tue, May 19, 2020 at 11:32 AM Marco Elver <elver@google.com> wrote:
>
> This fixes the problem:
> https://lkml.kernel.org/r/20200519182459.87166-1-elver@google.com
>
> I suppose there are several things that happened that caused the above
> bisected changes to trigger this. Hard to say how exactly the above
> bisected changes caused this to manifest, because during early boot
> (while uninitialized) KASAN may just randomly enter kasan_report()
> before the branch (annotated with likely(), which is caught by the
> branch tracer) prevents it from actually generating a report. However,
> if it goes branch tracer -> KASAN -> branch tracers -> KASAN ..., then
> we crash. If I had to guess some combination of different code gen,
> different stack and/or data usage. So all the above bisected changes
> (AFAIK) were red herrings. :-)

Thanks for chasing to resolution.  Consider using a variable to store
a list of flags, as that code (before your patch) invokes the compiler
multiple times to answer the same question.
-- 
Thanks,
~Nick Desaulniers