linux-kernel - CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled

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Message-ID: <20160130003645.GA30158@kmu-tp-x230>
Date:	Sat, 30 Jan 2016 03:36:45 +0300
From:	Mike Krinkin <krinkin.m.u@...il.com>
To:	aryabinin@...tuozzo.com
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled

Hi,

option CONFIG_UBSAN_ALIGNMENT breaks x86-64 kernel with lockdep enabled,
i. e kernel with CONFIG_UBSAN_ALIGNMENT fails to load without even any
error message.

The problem is that ubsan callbacks use spinlocks and might be called
before lockdep is initialized. Particularly this line in the
reserve_ebda_region function causes problem:

lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);

If i put lockdep_init() before reserve_ebda_region call in
x86_64_start_reservations kernel loads well. Since CONFIG_UBSAN_ALIGNMENT
isn't useful for x86 anyway it might be better to disable this option for
x86 arch?