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    SubjectRe: [PATCH v5] acpi: Fix CPU hot removal problem
    Hi Len and Chen Gong,
    What's you opinion on this patch?

    Thanks.

    ---
    canquan.shen
    2011-10-18

    On 2011/9/30 13:20, Bjorn Helgaas wrote:
    > On Wed, Sep 28, 2011 at 9:41 PM, canquan.shen<shencanquan@huawei.com> wrote:
    >>
    >> On 2011/9/29 11:14, Chen Gong wrote:
    >>>
    >>> 于 2011/9/24 14:10, canquan.shen 写道:
    >>>>
    >>>> We run linux as a guest in Xen environment. When we used the xen tools
    >>>> (xm vcpu-set<n>) to hot add and remove vcpu to and from the guest, we
    >>>> encountered the failure on vcpu removal. We found the reason is that it
    >>>> did't go to really remove cpu in the cpu removal code path.
    >>>>
    >>>> This patch adds acpi_bus_hot_remove_device in
    >>>> acpi_process_hotplug_notify to
    >>>> fix this issue. With this patch, it works fine for us.
    >>>>
    >>>> Signed-off-by: Canquan Shen<shencanquan@huawei.com>
    >>>> ---
    >>>> drivers/acpi/processor_driver.c | 13 +------------
    >>>> drivers/acpi/scan.c | 4 ++--
    >>>> include/acpi/acpi_bus.h | 1 +
    >>>> 3 files changed, 4 insertions(+), 14 deletions(-)
    >>>>
    >>>> diff --git a/drivers/acpi/processor_driver.c
    >>>> b/drivers/acpi/processor_driver.c
    >>>> index a4e0f1b..8429688 100644
    >>>> --- a/drivers/acpi/processor_driver.c
    >>>> +++ b/drivers/acpi/processor_driver.c
    >>>> @@ -665,18 +665,7 @@ static void
    >>>> acpi_processor_hotplug_notify(acpi_handle handle,
    >>>> case ACPI_NOTIFY_EJECT_REQUEST:
    >>>> ACPI_DEBUG_PRINT((ACPI_DB_INFO,
    >>>> "received ACPI_NOTIFY_EJECT_REQUEST\n"));
    >>>> -
    >>>> - if (acpi_bus_get_device(handle,&device)) {
    >>>> - printk(KERN_ERR PREFIX
    >>>> - "Device don't exist, dropping EJECT\n");
    >>>> - break;
    >>>> - }
    >>>> - pr = acpi_driver_data(device);
    >>>> - if (!pr) {
    >>>> - printk(KERN_ERR PREFIX
    >>>> - "Driver data is NULL, dropping EJECT\n");
    >>>> - return;
    >>>> - }
    >>>> + acpi_bus_hot_remove_device(handle);
    >>>
    >>> As the description in __acpi_os_execute(in acpi_os_hotplug_execute),
    >>> /*
    >>> * We can't run hotplug code in keventd_wq/kacpid_wq/kacpid_notify_wq
    >>> * because the hotplug code may call driver .remove() functions,
    >>> * which invoke flush_scheduled_work/acpi_os_wait_events_complete
    >>> * to flush these workqueues.
    >>> */
    >>> If so, why not using following call:
    >>>
    >>> acpi_os_hotplug_execute(acpi_bus_hot_remove_device, acpi_device->handle);
    >>>
    >>>
    >>
    >> It is ok if using the acpi_os_hotplug_execute. but it is complex and more time for removal cpu because it is add to queue and some time the work will be called.
    >> I think that it is clear to call directly acpi_bus_hot_remove_device function in acpi_processor_hotplug_notify.
    >
    > Chen, you're right that the CPU hot-remove notifier is running in a
    > workqueue, we are proposing that the notifier call
    > acpi_bus_hot_remove_device(), which calls the .remove() method, and
    > that can cause a deadlock if .remove() waits for the workqueue to be
    > flushed.
    >
    > The usual way this deadlock happens is when the .remove() method uses
    > acpi_remove_notify_handler(). But the processor_driver .remove()
    > method doesn't do anything with notify handler registration; the
    > handler is registered/unregistered by the module init/exit functions.
    > So I don't think we need to use acpi_os_hotplug_execute() in this case
    > because I don't think there's a risk of deadlock.
    >
    > However, if new CPU devices appear in the namespace after module-init,
    > I don't think processor_driver will handle them correctly. This looks
    > like just another artifact of our screwed-up ACPI hotplug handling.
    >
    > Bjorn
    >
    > .
    >


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