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

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Date:   Wed, 05 May 2021 10:47:07 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, 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

* Peter Zijlstra:

> On Tue, May 04, 2021 at 08:39:45PM -0700, Andi Kleen wrote:
>> From: Andi Kleen <andi@...stfloor.org>
>> 
>> The scheduler initialization code checks that the scheduling
>> classes are consecutive in memory by comparing the end
>> addresses with the next address.
>> 
>> Technically in ISO C comparing symbol addresseses outside different objects
>> is undefined. With LTO gcc 10 tries to exploits this and creates an
>> unconditional BUG_ON in the scheduler initialization, resulting
>> in a boot hang.
>> 
>> 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?

Context:

  <https://lore.kernel.org/lkml/20210505033945.1282851-1-ak@linux.intel.com/>

Obviously, GCC doesn't do this in general.  Would you please provide a
minimal test case?

Thanks,
Florian