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

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Date:	Mon, 27 Oct 2008 10:54:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH resend] x86/proc: fix /proc/cpuinfo cpu offline bug


* Lai Jiangshan <laijs@...fujitsu.com> wrote:

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

fix is already upstream - see below.

	Ingo

-------------------->
Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=bc8bcc79ea4203c7d04309f1307ab88c86f0b0cf
Commit:     bc8bcc79ea4203c7d04309f1307ab88c86f0b0cf
Parent:     35af28219e684a36cc8b1ff456c370ce22be157d
Author:     Lai Jiangshan <laijs@...fujitsu.com>
AuthorDate: Wed Oct 22 12:42:30 2008 +0800
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Wed Oct 22 14:29:37 2008 +0200

    x86/proc: fix /proc/cpuinfo cpu offline bug
    
    Impact: fix missing CPUs in /proc/cpuinfo after CPU hotunplug/hotreplug
    
    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@...fujitsu.com>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/kernel/cpu/proc.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

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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