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Date:   Tue, 19 May 2020 19:47:36 -0700
From:   Nathan Chancellor <natechancellor@gmail.com>
To:     Qian Cai <cai@lca.pw>
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@googlegroups.com
Subject: Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE
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Message-ID: <20200520024736.GA854786@ubuntu-s3-xlarge-x86>
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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> wrote:
> > 
> > Yes, it's unfortunate, but we have to stop making major concessions just
> > 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 version. 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. Anyway, it is just some other more bugs I have to deal with, and I don’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).

Cheers,
Nathan