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From:   Alexander Potapenko <glider@google.com>
Date:   Fri, 10 Jan 2020 09:37:06 +0100
Message-ID: <CAG_fn=UU0fuws59L8Bp8DEVhH+X6xRaanwuRrzy-HNdrVpqJmg@mail.gmail.com>
Subject: Re: INFO: rcu detected stall in sys_kill
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Daniel Axtens <dja@axtens.net>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        syzbot <syzbot+de8d933e7d153aa0c1bb@syzkaller.appspotmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Thu, Jan 9, 2020 at 6:39 PM 'Nick Desaulniers' via kasan-dev
<kasan-dev@googlegroups.com> wrote:
>
> On Thu, Jan 9, 2020 at 9:23 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> >
> > On Thu, Jan 9, 2020 at 6:17 PM Nick Desaulniers <ndesaulniers@google.co=
m> wrote:
> > > I disabled loop unrolling and loop unswitching in LLVM when the loop
> > > contained asm goto in:
> > > https://github.com/llvm/llvm-project/commit/c4f245b40aad7e8627b37a8bf=
1bdcdbcd541e665
> > > I have a fix for loop unrolling in:
> > > https://reviews.llvm.org/D64101
> > > that I should dust off. I haven't looked into loop unswitching yet.
> >
> > c4f245b40aad7e8627b37a8bf1bdcdbcd541e665 is in the range between the
> > broken compiler and the newer compiler that seems to work, so I would
> > assume that that commit fixes this.
> > We will get the final stamp from syzbot hopefully by tomorrow.
>
> How often do you refresh the build of Clang in syzbot? Is it manual? I
> understand the tradeoffs of living on the tip of the spear, but
> c4f245b40aad7e8627b37a8bf1bdcdbcd541e665 is 6 months old.  So upstream
> LLVM could be regressing more often, and you wouldn't notice for 1/2 a
> year or more. :-/
KMSAN used to be the only user of Clang on syzbot, so I didn't bother too o=
ften.
Now that there are other users, we'll need a better strategy.
Clang revisions I've been picking previously came from Chromium's
Clang distributions. This is nice, because Chromium folks usually pick
a revision that has been extensively tested at Google already, plus
they make sure Chromium tests also pass.
They don't roll the compiler often, however (typically once a month or
two, but this time there were holidays, plus some nasty breakages).
> --
> Thanks,
> ~Nick Desaulniers
>
> --
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l.gmail.com.


--=20
Alexander Potapenko
Software Engineer

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