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Date:   Wed, 5 May 2021 08:46:19 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     linux-kernel@vger.kernel.org, Andi Kleen <andi@firstfloor.org>,
        linux-toolchains@vger.kernel.org
Subject: Re: [PATCH] sched: Work around undefined behavior in sched class
 checking
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On Tue, May 04, 2021 at 08:39:45PM -0700, Andi Kleen wrote:
> From: Andi Kleen <andi@firstfloor.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?