USB: fix race between root-hub wakeup & controller suspend

This patch (as1395) adds code to hcd_pci_suspend() for handling wakeup
races.  This is another general race pattern, similar to the "open
vs. unregister" race we're all familiar with.  Here, the race is
between suspending a device and receiving a wakeup request from one of
the device's suspended children.

In particular, if a root-hub wakeup is requested at about the same
time as the corresponding USB controller is suspended, and if the
controller is enabled for wakeup, then the controller should either
fail to suspend or else wake right back up again.

During system sleep this won't happen very much, especially since host
controllers generally aren't enabled for wakeup during sleep.  However
it is definitely an issue for runtime PM.  Something like this will be
needed to prevent the controller from autosuspending while waiting for
a root-hub resume to take place.  (That is, in fact, the common case,
for which there is an extra test.)
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/hcd-pci.c

@@ -388,8 +388,20 @@ static int hcd_pci_suspend(struct device *dev)
 	if (hcd->driver->pci_suspend) {
 		bool	do_wakeup = device_may_wakeup(dev);
 
+		/* Optimization: Don't suspend if a root-hub wakeup is
+		 * pending and it would cause the HCD to wake up anyway.
+		 */
+		if (do_wakeup && HCD_WAKEUP_PENDING(hcd))
+			return -EBUSY;
 		retval = hcd->driver->pci_suspend(hcd, do_wakeup);
 		suspend_report_result(hcd->driver->pci_suspend, retval);
+
+		/* Check again in case wakeup raced with pci_suspend */
+		if (retval == 0 && do_wakeup && HCD_WAKEUP_PENDING(hcd)) {
+			if (hcd->driver->pci_resume)
+				hcd->driver->pci_resume(hcd, false);
+			retval = -EBUSY;
+		}
 		if (retval)
 			return retval;
 	}

drivers/usb/core/hcd.c

@@ -1940,6 +1940,7 @@ int hcd_bus_resume(struct usb_device *rhdev, pm_message_t msg)
 
 	dev_dbg(&rhdev->dev, "usb %s%s\n",
 			(msg.event & PM_EVENT_AUTO ? "auto-" : ""), "resume");
+	clear_bit(HCD_FLAG_WAKEUP_PENDING, &hcd->flags);
 	if (!hcd->driver->bus_resume)
 		return -ENOENT;
 	if (hcd->state == HC_STATE_RUNNING)
@@ -1993,8 +1994,10 @@ void usb_hcd_resume_root_hub (struct usb_hcd *hcd)
 	unsigned long flags;
 
 	spin_lock_irqsave (&hcd_root_hub_lock, flags);
-	if (hcd->rh_registered)
+	if (hcd->rh_registered) {
+		set_bit(HCD_FLAG_WAKEUP_PENDING, &hcd->flags);
 		queue_work(pm_wq, &hcd->wakeup_work);
+	}
 	spin_unlock_irqrestore (&hcd_root_hub_lock, flags);
 }
 EXPORT_SYMBOL_GPL(usb_hcd_resume_root_hub);

include/linux/usb/hcd.h

@@ -98,6 +98,7 @@ struct usb_hcd {
 #define HCD_FLAG_SAW_IRQ		1
 #define HCD_FLAG_POLL_RH		2	/* poll for rh status? */
 #define HCD_FLAG_POLL_PENDING		3	/* status has changed? */
+#define HCD_FLAG_WAKEUP_PENDING		4	/* root hub is resuming? */
 
 	/* The flags can be tested using these macros; they are likely to
 	 * be slightly faster than test_bit().
@@ -106,6 +107,7 @@ struct usb_hcd {
 #define HCD_SAW_IRQ(hcd)	((hcd)->flags & (1U << HCD_FLAG_SAW_IRQ))
 #define HCD_POLL_RH(hcd)	((hcd)->flags & (1U << HCD_FLAG_POLL_RH))
 #define HCD_POLL_PENDING(hcd)	((hcd)->flags & (1U << HCD_FLAG_POLL_PENDING))
+#define HCD_WAKEUP_PENDING(hcd)	((hcd)->flags & (1U << HCD_FLAG_WAKEUP_PENDING))
 
 	/* Flags that get set only during HCD registration or removal. */
 	unsigned		rh_registered:1;/* is root hub registered? */