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

[canquan.shen]

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@xxxxxxxxxx>  wrote:
> > On 2011/9/29 11:14, Chen Gong wrote:
> > > ä 2011/9/24 14:10, canquan.shen åé:
> > > > 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@xxxxxxxxxx>
> > > > ---
> > > > 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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