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Date:   Tue, 16 Jun 2020 18:42:02 +0200
From:   Marco Elver <elver@google.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org, josh@joshtriplett.org,
        rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, shuah@kernel.org, rcu@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Subject: Re: [PATCH] torture: Pass --kmake-arg to all make invocations
Message-ID: <20200616164202.GA208325@google.com>
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On Tue, Jun 16 2020 at 09:05AM -0700, Paul E. McKenney wrote:

> On Tue, Jun 16, 2020 at 11:49:24AM +0200, Marco Elver wrote:
> > We need to pass the arguments provided to --kmake-arg to all make
> > invocations. In particular, the make invocations generating the configs
> > need to see the final make arguments, e.g. if config variables depend on
> > particular variables that are passed to make.
> > 
> > For example, when using '--kcsan --kmake-arg CC=clang-11', we would lose
> > CONFIG_KCSAN=y due to 'make oldconfig' not seeing that we want to use a
> > compiler that supports KCSAN.
> > 
> > Signed-off-by: Marco Elver <elver@google.com>
> 
> Queued and pushed, thank you!
> 
> Would the following patch make sense, at least until such time
> as some other compiler supports KCSAN?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 88bcaa730b6d40ddf69b09ed6f0a14803d087d99
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Tue Jun 16 09:02:34 2020 -0700
> 
>     torture: Make --kcsan default to using Clang 11
>     
>     Currently, Clang 11 is the only compiler that can support KCSAN.
>     Therefore, as a convenience to the KCSAN user, this commit causes
>     --kcsan to specify Clang 11 unless a "CC=" argument was already
>     specified via the --kmake-arg argument.

As soon as more compilers support KCSAN (e.g. clang-12, etc...) we run
the risk of actually inconveniencing ourselves more because then we
really need to say '--kmake-arg CC=clang-1X' to not use the old
compiler. Or revert this in time.

My command-line looks more like this right now:

	kvm.sh ... --kmake-arg "CC="${HOME}/local/<gcc-or-clang>-11.kcsan/local/bin/<gcc-or-clang>" ...

I think the safer alternative would be to error if CONFIG_KCSAN=y is not
in the config, and simply suggest "Did you forget to switch your
compiler with '--kmake-arg CC=<cc-that-supports-kcsan>'?" (of course, a
'gcc' in $PATH that supports KCSAN would also be fine -- see below).
Eventually, when the default compilers support KCSAN, this will resolve
itself gracefully.

Also, I'm going to send a series later this week to re-enable GCC
support. ;-)

Thanks,
-- Marco