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From:   Qian Cai <cai@lca.pw>
Date:   Tue, 19 May 2020 23:16:24 -0400
Message-ID: <CAG=TAF4M5s1kQ98ys_YCgRS9WqjV_9KEbPCFiS71MA_QK8epdA@mail.gmail.com>
Subject: Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE variants
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Marco Elver <elver@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Will Deacon <will@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Tue, May 19, 2020 at 10:47 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Tue, May 19, 2020 at 10:28:41PM -0400, Qian Cai wrote:
> >
> >
> > > On May 19, 2020, at 6:05 PM, Thomas Gleixner <tglx@linutronix.de> wro=
te:
> > >
> > > Yes, it's unfortunate, but we have to stop making major concessions j=
ust
> > > because tools are not up to the task.
> > >
> > > We've done that way too much in the past and this particular problem
> > > clearly demonstrates that there are limits.
> > >
> > > Making brand new technology depend on sane tools is not asked too
> > > much. And yes, it's inconvenient, but all of us have to build tools
> > > every now and then to get our job done. It's not the end of the world=
.
> > >
> > > Building clang is trivial enough and pointing the make to the right
> > > compiler is not rocket science either.
> >
> > Yes, it all make sense from that angle. On the other hand, I want to be=
 focus on kernel rather than compilers by using a stable and rocket-solid v=
ersion. Not mentioned the time lost by compiling and properly manage my own=
 toolchain in an automated environment, using such new version of compilers=
 means that I have to inevitably deal with compiler bugs occasionally. Anyw=
ay, it is just some other more bugs I have to deal with, and I don=E2=80=99=
t have a better solution to offer right now.
>
> Hi Qian,
>
> Shameless plug but I have made a Python script to efficiently configure
> then build clang specifically for building the kernel (turn off a lot of
> different things that the kernel does not need).
>
> https://github.com/ClangBuiltLinux/tc-build
>
> I added an option '--use-good-revision', which uses an older master
> version (basically somewhere between clang-10 and current master) that
> has been qualified against the kernel. I currently update it every
> Linux release but I am probably going to start doing it every month as
> I have written a pretty decent framework to ensure that nothing is
> breaking on either the LLVM or kernel side.
>
> $ ./build-llvm.py --use-good-revision
>
> should be all you need to get off the ground and running if you wanted
> to give it a shot. The script is completely self contained by default so
> it won't mess with the rest of your system. Additionally, leaving off
> '--use-good-revision' will just use the master branch, which can
> definitely be broken but not as often as you would think (although I
> totally understand wanting to focus on kernel regressions only).

Great, thanks. I'll try it in a bit.