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    SubjectRe: [PATCH 1/2] acpi : cpu hot-remove returns error number when cpu_down() fails
    On 07/09/2012 08:01 AM, Yasuaki Ishimatsu wrote:
    > Hi Srivatsa,
    >
    > Thank you for your reviewing.
    >
    > 2012/07/06 18:51, Srivatsa S. Bhat wrote:
    >> On 07/06/2012 08:46 AM, Yasuaki Ishimatsu wrote:
    >>> Even if cpu_down() fails, acpi_processor_remove() continues to remove the cpu.
    >>
    >> Ouch!
    >>
    >>> But in this case, it should return error number since some process may run on
    >>> the cpu. If the cpu has a running process and the cpu is turned the power off,
    >>> the system cannot work well.
    >>>
    >>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
    >>>
    >>> ---
    >>> drivers/acpi/processor_driver.c | 18 ++++++++++++------
    >>> 1 file changed, 12 insertions(+), 6 deletions(-)
    >>>
    >>> Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
    >>> ===================================================================
    >>> --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c 2012-06-25 04:53:04.000000000 +0900
    >>> +++ linux-3.5-rc4/drivers/acpi/processor_driver.c 2012-07-05 21:02:58.711285382 +0900
    >>> @@ -610,7 +610,7 @@ err_free_pr:
    >>> static int acpi_processor_remove(struct acpi_device *device, int type)
    >>> {
    >>> struct acpi_processor *pr = NULL;
    >>> -
    >>> + int ret;
    >>>
    >>> if (!device || !acpi_driver_data(device))
    >>> return -EINVAL;
    >>> @@ -621,8 +621,9 @@ static int acpi_processor_remove(struct
    >>> goto free;
    >>>
    >>> if (type == ACPI_BUS_REMOVAL_EJECT) {
    >>> - if (acpi_processor_handle_eject(pr))
    >>> - return -EINVAL;
    >>> + ret = acpi_processor_handle_eject(pr);
    >>> + if (ret)
    >>> + return ret;
    >>> }
    >>>
    >>> acpi_processor_power_exit(pr, device);
    >>> @@ -841,12 +842,17 @@ static acpi_status acpi_processor_hotadd
    >>>
    >>> static int acpi_processor_handle_eject(struct acpi_processor *pr)
    >>> {
    >>> - if (cpu_online(pr->id))
    >>> - cpu_down(pr->id);
    >>> + int ret;
    >>> +
    >>> + if (cpu_online(pr->id)) {
    >>> + ret = cpu_down(pr->id);
    >>> + if (ret)
    >>> + return ret;
    >>> + }
    >>>
    >>
    >> Strictly speaking, this is not thorough enough. What prevents someone
    >> from onlining that same cpu again, at this point?
    >> So, IMHO, you need to wrap the contents of this function inside a
    >> get_online_cpus()/put_online_cpus() block, to prevent anyone else
    >> from messing with CPU hotplug at the same time.
    >
    > If I understand your comment by mistake, please let me know.
    > If the contents is wrapped a inside get_online_cpus()/put_online_cpus() block
    > as below, cpu_down() will stop since cpu_down() calls cpu_hotplug_begin() and
    > cpu_hotplug_begin() waits for cpu_hotplug.refcount to become 0.
    >

    You are right. Sorry, I overlooked that.

    > + get_online_cpus()
    > + if (cpu_online(pr->id)) {
    > + ret = cpu_down(pr->id);
    > + if (ret)
    > + return ret;
    > + }
    > + put_online_cpus()
    >
    > I think following patch can prevent it correctly. How about the patch?
    >
    > ---
    > drivers/acpi/processor_driver.c | 12 ++++++++++++
    > kernel/cpu.c | 8 +++++---
    > 2 files changed, 17 insertions(+), 3 deletions(-)
    >
    > Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
    > ===================================================================
    > --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c 2012-07-09 09:59:01.280211202 +0900
    > +++ linux-3.5-rc4/drivers/acpi/processor_driver.c 2012-07-09 11:05:34.559859236 +0900
    > @@ -844,14 +844,26 @@ static int acpi_processor_handle_eject(s
    > {
    > int ret;
    >
    > +retry:
    > if (cpu_online(pr->id)) {
    > ret = cpu_down(pr->id);
    > if (ret)
    > return ret;
    > }
    >
    > + get_online_cpus();
    > + /*
    > + * Someone might online the cpu again at this point. So we check that
    > + * cpu has been onlined or not. If cpu is online, we try to offline
    > + * the cpu again.
    > + */
    > + if (cpu_online(pr->id)) {

    How about this:
    if (unlikely(cpu_online(pr->id)) {
    since the probability of this happening is quite small...

    > + put_online_cpus();
    > + goto retry;
    > + }
    > arch_unregister_cpu(pr->id);
    > acpi_unmap_lsapic(pr->id);
    > + put_online_cpus();
    > return ret;
    > }

    This retry logic doesn't look elegant, but I don't see any better method :-(

    > #else
    > Index: linux-3.5-rc4/kernel/cpu.c
    > ===================================================================
    > --- linux-3.5-rc4.orig/kernel/cpu.c 2012-07-09 09:59:01.280211202 +0900
    > +++ linux-3.5-rc4/kernel/cpu.c 2012-07-09 09:59:02.903190965 +0900
    > @@ -343,11 +343,13 @@ static int __cpuinit _cpu_up(unsigned in
    > unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
    > struct task_struct *idle;
    >
    > - if (cpu_online(cpu) || !cpu_present(cpu))
    > - return -EINVAL;
    > -
    > cpu_hotplug_begin();
    >
    > + if (cpu_online(cpu) || !cpu_present(cpu)) {
    > + ret = -EINVAL;
    > + goto out;
    > + }
    > +

    Firstly, why is this change needed?
    Secondly, if the change is indeed an improvement, then why is it
    in _this_ patch? IMHO, in that case it should be part of a separate patch.

    Coming back to my first point, I don't see why this hunk is needed. We
    already take the cpu_add_remove_lock (cpu_maps_update_begin/end) before
    checking the status of the cpu (online or present). And all hotplug
    operations (cpu_up/cpu_down/disable|enable_nonboot_cpus) go through that
    lock. Isn't that enough? Or am I missing something?

    > idle = idle_thread_get(cpu);
    > if (IS_ERR(idle)) {
    > ret = PTR_ERR(idle);
    >

    Regards,
    Srivatsa S. Bhat



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