[PATCH] x86/proc: fix /proc/cpuinfo cpu offline bug

[PATCH] x86/proc: fix /proc/cpuinfo cpu offline bug

[Lai Jiangshan]

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");

2) rebuilt kernel & reboot
3) offline cpu#1
4) cat /proc/cpuinfo
   cpu#2 and cpu#3 .... cannot be shown in /proc/cpuinfo

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
index a26c480..01b1244 100644
--- 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);
 }
 

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

[Alexey Dobriyan]

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?

> 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);
>  }

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

[Lai Jiangshan]

On Fri, Oct 17, 2008 at 7:41 PM, Alexey Dobriyan <adobriyan@gmail.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);
>>  }
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
>

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

[Lai Jiangshan]

Hi, Ingo and Alexey,

I have changed my changelog. please review again.

Thanks, Lai.

Lai Jiangshan wrote:
> On Fri, Oct 17, 2008 at 7:41 PM, Alexey Dobriyan <adobriyan@gmail.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);
>>>  }
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>>
> 
> 
> 

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

[Lai Jiangshan]

Lai Jiangshan wrote:
> Hi, Ingo and Alexey,
> 
> I have changed my changelog. please review again.
> 
> Thanks, Lai.
> 


In my test, I found that if a cpu has been offline,
the next cpus may not be shown in the /proc/cpuinfo.

if one read() cannot consume the whole /proc/cpuinfo,
c_start() will be called again in the next read() calls.
And *pos has been increased by 1 by the caller(seq_read()).
if this time the cpu#*pos is offline, c_start() will return
NULL, and the next cpus can not be shown.

this fix use next_cpu_nr(*pos - 1, cpu_online_map) to
search the next unshown cpu.

the most easy way to reproduce this bug:
1) offline cpu#1             (cpu#0 is online)
2) dd ibs=2 if=/proc/cpuinfo
   the result is that only cpu#0 is shown.
   cpu#2 and cpu#3 .... cannot be shown in /proc/cpuinfo
   it's bug.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
index a26c480..01b1244 100644
--- 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);
 }