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    SubjectRe: [PATCH v5] acpi: Fix CPU hot removal problem
    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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