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

From: Florian Weimer <fweimer@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>,
	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
Date: Wed, 05 May 2021 10:47:07 +0200	[thread overview]
Message-ID: <875yzxh8j8.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <YJI/OwoflyY2IXvf@hirez.programming.kicks-ass.net> (Peter Zijlstra's message of "Wed, 5 May 2021 08:46:19 +0200")

* Peter Zijlstra:

> 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?

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