linux-kernel - Re: 2.6.25-mm1: not looking good

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Message-ID: <20080418092254.GA20661@elte.hu>
Date:	Fri, 18 Apr 2008 11:22:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, James Morris <jmorris@...ei.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-pm@...ts.linux-foundation.org, Greg KH <greg@...ah.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Pavel Machek <pavel@....cz>,
	Jack Steiner <steiner@....com>, Mike Travis <travis@....com>,
	Alan Mayer <ajm@....com>
Subject: Re: 2.6.25-mm1: not looking good


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> WARNING: at arch/x86/kernel/genapic_64.c:86 read_apic_id+0x31/0x67()
>
> [<ffffffff803523e6>] ? debug_smp_processor_id+0x32/0xc4
> [<ffffffff8021ede5>] read_apic_id+0x31/0x67
> [<ffffffff8066f7f2>] verify_local_APIC+0xa7/0x163
> [<ffffffff8066e837>] native_smp_prepare_cpus+0x1ed/0x301
> [<ffffffff80669ab2>] kernel_init+0x5a/0x276

that came in via the UV-APIC patchset but the warning is entirely 
harmless. At that point we've got a single CPU running only so 
preemption of that code to another CPU is not possible.

native_smp_prepare_cpus() should probably just disable preemption, that 
way we could remove all those ugly preempt disable-enable calls from the 
called functions - per the patch below. (not boot tested yet - might 
provoke atomic-scheduling warnings if i forgot about some schedule point 
in this rather large codepath)

	Ingo

------------------->
Subject: x86: disable preemption in native_smp_prepare_cpus
From: Ingo Molnar <mingo@...e.hu>
Date: Fri Apr 18 11:07:10 CEST 2008

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/kernel/smpboot.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-x86.q/arch/x86/kernel/smpboot.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/smpboot.c
+++ linux-x86.q/arch/x86/kernel/smpboot.c
@@ -1181,6 +1181,7 @@ static void __init smp_cpu_index_default
  */
 void __init native_smp_prepare_cpus(unsigned int max_cpus)
 {
+	preempt_disable();
 	nmi_watchdog_default();
 	smp_cpu_index_default();
 	current_cpu_data = boot_cpu_data;
@@ -1237,6 +1238,7 @@ void __init native_smp_prepare_cpus(unsi
 	printk(KERN_INFO "CPU%d: ", 0);
 	print_cpu_info(&cpu_data(0));
 	setup_boot_clock();
+	preempt_enable();
 }
 /*
  * Early setup to make printk work.
--
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