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@xxxxxxxxxxxxxx>
> 
> 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?