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

This is a discussion on [PATCH resend] x86/proc: fix /proc/cpuinfo cpu offline bug - Kernel ; 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 file, c_start() will be called again in the next read() ...

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

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

    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 file,
    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 uses next_cpu_nr(*pos - 1, cpu_online_map) to
    search the next unshown cpu.

    the way to reproduce this bug:
    (cpu#0 cpu#1 cpu#2 must be online before test)

    step 1) offline cpu#1
    step 2) dd ibs=2 if=/proc/cpuinfo

    now, only cpu#0 's cpuinfo is shown.
    cpu#2 and cpu#3 .... cannot be shown in /proc/cpuinfo
    it's a 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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  2. Re: [PATCH resend] x86/proc: fix /proc/cpuinfo cpu offline bug


    * 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.


    fix is already upstream - see below.

    Ingo

    -------------------->
    Gitweb: http://git.kernel.org/git/?p=linux/k...7ab88c86f0b0cf
    Commit: bc8bcc79ea4203c7d04309f1307ab88c86f0b0cf
    Parent: 35af28219e684a36cc8b1ff456c370ce22be157d
    Author: Lai Jiangshan
    AuthorDate: Wed Oct 22 12:42:30 2008 +0800
    Committer: Ingo Molnar
    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
    Signed-off-by: Ingo Molnar
    ---
    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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