linux-kernel - Re: [PATCH] sched: Work around undefined behavior in sched class checking

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Date:   Wed, 5 May 2021 17:34:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] sched: Work around undefined behavior in sched class
 checking

On Wed, May 05, 2021 at 07:34:42AM -0700, Andi Kleen wrote:
> > > Use RELOC_HIDE to make this work. This hides the symbols from gcc,
> > > so the optimizer won't make these assumption. I also split
> > > the BUG_ONs in multiple.
> > 
> > Urgh, that insanity again :/ Can't we pretty please get a GCC flag to
> > disable that?
> 
> Even if that was done (I could totally see the gcc people pushing back on this;
> why should they add special flags just for Linux developers not understanding
> ISO-C?)

I understand C fine, I just don't agree with it. I also want to
explicitly define as much UB as is possible, because UB is just utter
garbage.

So just like we do with -fwrapv and others, add more knobs that
explictly define away UB. Less UB is more better. This being C it's
unlikely we'll ever get to no UB, but we should damn well try :-)

> you would still need the fix for already shipping compilers.

Yes, there is that.