linux-kernel - RE: [PATCH -next] ia64/sn: fix percpu warnings

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Message-ID: <57C9024A16AD2D4C97DC78E552063EA3E34C0629@orsmsx505.amr.corp.intel.com>
Date:	Wed, 28 Oct 2009 15:23:30 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	akpm <akpm@...ux-foundation.org>
Subject: RE: [PATCH -next] ia64/sn: fix percpu warnings

> I just tried an x86_64 build with a module also modified to access
> softirq_work_list.  It did NOT get a CRC error.  So the x86_64
> toolchain doesn't seem to have the same issue as ia64.

Ok.  x86 doesn't see this because the defconfig has
  CONFIG_MODVERSIONS is not set

If I turn MODVERSION off, then the problem disappears for me.

If I turn MODVERSIONS on for x86 (and fudge a driver to make use of
softirq_work_list), then x86 gets the CRC warning too.


So this isn't a tool chain problem. Neither x86 nor ia64
can handle exported per-cpu array objects when CONFIG_MODVERSIONS
is set.

Looking at the __crc symbols in the vmlinux for x86 with
CONFIG_MODVERSIONS=y I see:

000000006dcaeb88 A __crc_per_cpu__kernel_stack
00000000b3994c7a A __crc_per_cpu__kstat
00000000d917c158 A __crc_per_cpu__node_number
                 w __crc_per_cpu__softirq_work_list
0000000036a1f502 A __crc_per_cpu__softnet_data
0000000057adf756 A __crc_per_cpu__this_cpu_off

which explains why "modpost" is unable to find a CRC.

Maybe the comments in <linux/module.h> are supposed to be a
clue? :

#ifdef CONFIG_MODVERSIONS
/* Mark the CRC weak since genksyms apparently decides not to
 * generate a checksums for some symbols */
#define __CRC_SYMBOL(sym, sec)                                  \
        extern void *__crc_##sym __attribute__((weak));         \
        static const unsigned long __kcrctab_##sym              \
        __used                                                  \
        __attribute__((section("__kcrctab" sec), unused))       \
        = (unsigned long) &__crc_##sym;

But not enough of a clue for me :-(


-Tony
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