USB: replace flush_workqueue with cancel_sync_work

This patch (as912) replaces a couple of calls to flush_workqueue()
with cancel_sync_work() and cancel_rearming_delayed_work(). Using a
more directed approach allows us to avoid some nasty deadlocks. The
prime example occurs when a first-level device (the parent is a root
hub) is removed while at the same time the root hub gets a remote
wakeup request. khubd would try to flush the autosuspend workqueue
while holding the root-hub's lock, and the remote-wakeup workqueue
routine would be waiting to lock the root hub.

The patch also reorganizes the power management portion of
usb_disconnect(), separating it out into its own routine. The
autosuspend workqueue entry is cancelled immediately instead of
waiting for the device's release routine. In addition,
synchronization with the autosuspend thread is carried out even for
root hubs (an oversight in the original code).

Signed-off-by: Alan Stern <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Greg KH <[e-mail]>
Cc: Mark Lord <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/usb/core/hcd.c

···
 	spin_unlock_irq (&hcd_root_hub_lock);
 
 #ifdef CONFIG_PM
-	flush_workqueue(ksuspend_usb_wq);
+	cancel_work_sync(&hcd->wakeup_work);
 #endif
 
 	mutex_lock(&usb_bus_list_lock);

drivers/usb/core/hub.c

···
 	}
 }
 
+#ifdef	CONFIG_USB_SUSPEND
+
+static void usb_stop_pm(struct usb_device *udev)
+{
+	/* Synchronize with the ksuspend thread to prevent any more
+	 * autosuspend requests from being submitted, and decrement
+	 * the parent's count of unsuspended children.
+	 */
+	usb_pm_lock(udev);
+	if (udev->parent && !udev->discon_suspended)
+		usb_autosuspend_device(udev->parent);
+	usb_pm_unlock(udev);
+
+	/* Stop any autosuspend requests already submitted */
+	cancel_rearming_delayed_work(&udev->autosuspend);
+}
+
+#else
+
+static inline void usb_stop_pm(struct usb_device *udev)
+{ }
+
+#endif
+
 /**
  * usb_disconnect - disconnect a device (usbcore-internal)
  * @pdev: pointer to device being disconnected
···
 	*pdev = NULL;
 	spin_unlock_irq(&device_state_lock);
 
-	/* Decrement the parent's count of unsuspended children */
-	if (udev->parent) {
-		usb_pm_lock(udev);
-		if (!udev->discon_suspended)
-			usb_autosuspend_device(udev->parent);
-		usb_pm_unlock(udev);
-	}
+	usb_stop_pm(udev);
 
 	put_device(&udev->dev);
 }

drivers/usb/core/usb.c

···
 
 	udev = to_usb_device(dev);
 
-#ifdef	CONFIG_USB_SUSPEND
-	cancel_delayed_work(&udev->autosuspend);
-	flush_workqueue(ksuspend_usb_wq);
-#endif
 	usb_destroy_configuration(udev);
 	usb_put_hcd(bus_to_hcd(udev->bus));
 	kfree(udev->product);