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SubjectRe: [PATCH] x86/proc: fix /proc/cpuinfo cpu offline bug
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 <>
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);

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