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From:   Marco Elver <elver@google.com>
Date:   Wed, 20 May 2020 09:28:20 +0200
Message-ID: <CANpmjNMiiDN0AueijLbkbhEX0vLc3xfPyA7kec5_T3Qku7wkMw@mail.gmail.com>
Subject: Re: [PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE variants
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Qian Cai <cai@lca.pw>, Thomas Gleixner <tglx@linutronix.de>,
        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 Wed, 20 May 2020 at 05:44, Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Tue, May 19, 2020 at 11:16:24PM -0400, Qian Cai wrote:
> > 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>=
 wrote:
> > > > >
> > > > > Yes, it's unfortunate, but we have to stop making major concessio=
ns just
> > > > > because tools are not up to the task.
> > > > >
> > > > > We've done that way too much in the past and this particular prob=
lem
> > > > > 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 too=
ls
> > > > > every now and then to get our job done. It's not the end of the w=
orld.
> > > > >
> > > > > Building clang is trivial enough and pointing the make to the rig=
ht
> > > > > compiler is not rocket science either.
> > > >
> > > > Yes, it all make sense from that angle. On the other hand, I want t=
o be focus on kernel rather than compilers by using a stable and rocket-sol=
id version. Not mentioned the time lost by compiling and properly manage my=
 own toolchain in an automated environment, using such new version of compi=
lers 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=E2=
=80=99t have a better solution to offer right now.
> > >
> > > Hi Qian,
> > >
> > > Shameless plug but I have made a Python script to efficiently configu=
re
> > > 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) tha=
t
> > > has been qualified against the kernel. I currently update it every
> > > Linux release but I am probably going to start doing it every month a=
s
> > > 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 wante=
d
> > > 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.
>
> Please let me know if there are any issues!
>
> Do note that in order to get support for Marco's series, you will need
> to have a version of LLVM that includes [1], which the current
> --use-good-revision does not. You can checkout that revision exactly
> through the '-b' ('--branch') parameter:
>
> $ ./build-llvm.py -b 5a2c31116f412c3b6888be361137efd705e05814
>
> I also see another patch in LLVM that concerns KCSAN [2] but that does
> not appear used in Marco's series. Still might be worth having available
> in your version of clang.
>
> I'll try to bump the hash that '--use-good-revision' uses soon. I might
> wait until 5.7 final so that I can do both at the same time like I
> usually do but we'll see how much time I have.
>
> [1]: https://github.com/llvm/llvm-project/commit/5a2c31116f412c3b6888be36=
1137efd705e05814
> [2]: https://github.com/llvm/llvm-project/commit/151ed6aa38a3ec6c01973b35=
f684586b6e1c0f7e

Thanks for sharing the script, this is very useful!

Note that [2] above is used, but optional:
https://lore.kernel.org/lkml/20200515150338.190344-5-elver@google.com/
It's not required for KCSAN to function correctly, but if it's
available it'll help find more data races with the default config.

Thanks,
-- Marco