linux-kernel - Re: [PATCH] x86/proc: fix /proc/cpuinfo cpu offline bug

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Date:	Fri, 17 Oct 2008 20:44:51 +0800
From:	"Lai Jiangshan" <eag0628@...il.com>
To:	"Alexey Dobriyan" <adobriyan@...il.com>
Cc:	"Lai Jiangshan" <laijs@...fujitsu.com>,
	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/proc: fix /proc/cpuinfo cpu offline bug

On Fri, Oct 17, 2008 at 7:41 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> On Fri, Oct 17, 2008 at 04:55:25PM +0800, Lai Jiangshan wrote:
>> In my test, I found that if a cpu has been offline,
>> the next cpus may not be shown in the /proc/cpuinfo.
>>
>> trivially reproduce this bug:
>>
>> 1) add these lines in the end of show_cpuinfo()
>>       if (m->size - m->count - 20 > 0)
>>               seq_printf(m, "%*s", (int)(m->size - m->count - 20), "show bug\n");
>
> What is it?
>
> Can you just show wrong /proc/cpuinfo ?
>
> Can someone with at least 4-way box please do so?

this is not wrong /proc/cpuinfo, this is an enlarged /proc/cpuinfo
this trivial example just use "show bug\n" as the enlarged content.

if you boot your machine with enough cpu, your /proc/cpuinfo
have been enlarged too, please use this:

#!/bin/sh
nr_cpus=16 #  i386
for ((i=1; i<nr_cpus; i++))
do
    echo 0 > /sys/devices/system/cpu/cpu$i/online
    cpus=$(grep processor /proc/cpuinfo | wc -l)
    (( cpus == nr_cpus -1 )) || break; # bug eccur
    echo 1 > /sys/devices/system/cpu/cpu$i/online
done
cat /proc/cpuinfo  # it shows this bug.

Lai

>
>> 3) offline cpu#1
>> 4) cat /proc/cpuinfo
>>    cpu#2 and cpu#3 .... cannot be shown in /proc/cpuinfo
>
>> --- a/arch/x86/kernel/cpu/proc.c
>> +++ b/arch/x86/kernel/cpu/proc.c
>> @@ -160,14 +160,16 @@ static void *c_start(struct seq_file *m, loff_t *pos)
>>  {
>>       if (*pos == 0)  /* just in case, cpu 0 is not the first */
>>               *pos = first_cpu(cpu_online_map);
>> -     if ((*pos) < nr_cpu_ids && cpu_online(*pos))
>> +     else
>> +             *pos = next_cpu_nr(*pos - 1, cpu_online_map);
>> +     if ((*pos) < nr_cpu_ids)
>>               return &cpu_data(*pos);
>>       return NULL;
>>  }
>>
>>  static void *c_next(struct seq_file *m, void *v, loff_t *pos)
>>  {
>> -     *pos = next_cpu(*pos, cpu_online_map);
>> +     (*pos)++;
>>       return c_start(m, pos);
>>  }
> --
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