usb-anchor: Delay usb_wait_anchor_empty_timeout wake up till completion is done

usb_wait_anchor_empty_timeout() should wait till the completion handler
has run. Both the zd1211rw driver and the uas driver (in its task mgmt) depend
on the completion handler having completed when usb_wait_anchor_empty_timeout()
returns, as they read state set by the completion handler after an
usb_wait_anchor_empty_timeout() call.

But __usb_hcd_giveback_urb() calls usb_unanchor_urb before calling the
completion handler. This is necessary as the completion handler may
re-submit and re-anchor the urb. But this introduces a race where the state
these drivers want to read has not been set yet by the completion handler
(this race is easily triggered with the uas task mgmt code).

I've considered adding an anchor_count to struct urb, which would be
incremented on anchor and decremented on unanchor, and then only actually
do the anchor / unanchor on 0 -> 1 and 1 -> 0 transtions, combined with
moving the unanchor call in hcd_giveback_urb to after calling the completion
handler. But this will only work if urb's are only re-anchored to the same
anchor as they were anchored to before the completion handler ran.

And at least one driver re-anchors to another anchor from the completion
handler (rtlwifi).

So I have come up with this patch instead, which adds the ability to
suspend wakeups of usb_wait_anchor_empty_timeout() waiters to the usb_anchor
functionality, and uses this in __usb_hcd_giveback_urb() to delay wake-ups
until the completion handler has run.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hcd.c

@@ -1652,6 +1652,7 @@ int usb_hcd_unlink_urb (struct urb *urb, int status)
 static void __usb_hcd_giveback_urb(struct urb *urb)
 {
 	struct usb_hcd *hcd = bus_to_hcd(urb->dev->bus);
+	struct usb_anchor *anchor = urb->anchor;
 	int status = urb->unlinked;
 	unsigned long flags;
 
@@ -1663,6 +1664,7 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
 
 	unmap_urb_for_dma(hcd, urb);
 	usbmon_urb_complete(&hcd->self, urb, status);
+	usb_anchor_suspend_wakeups(anchor);
 	usb_unanchor_urb(urb);
 
 	/* pass ownership to the completion handler */
@@ -1682,6 +1684,7 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
 	urb->complete(urb);
 	local_irq_restore(flags);
 
+	usb_anchor_resume_wakeups(anchor);
 	atomic_dec(&urb->use_count);
 	if (unlikely(atomic_read(&urb->reject)))
 		wake_up(&usb_kill_urb_queue);

drivers/usb/core/urb.c

@@ -138,13 +138,19 @@ void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor)
 }
 EXPORT_SYMBOL_GPL(usb_anchor_urb);
 
+static int usb_anchor_check_wakeup(struct usb_anchor *anchor)
+{
+	return atomic_read(&anchor->suspend_wakeups) == 0 &&
+		list_empty(&anchor->urb_list);
+}
+
 /* Callers must hold anchor->lock */
 static void __usb_unanchor_urb(struct urb *urb, struct usb_anchor *anchor)
 {
 	urb->anchor = NULL;
 	list_del(&urb->anchor_list);
 	usb_put_urb(urb);
-	if (list_empty(&anchor->urb_list))
+	if (usb_anchor_check_wakeup(anchor))
 		wake_up(&anchor->wait);
 }
 
@@ -845,6 +851,39 @@ void usb_unlink_anchored_urbs(struct usb_anchor *anchor)
 }
 EXPORT_SYMBOL_GPL(usb_unlink_anchored_urbs);
 
+/**
+ * usb_anchor_suspend_wakeups
+ * @anchor: the anchor you want to suspend wakeups on
+ *
+ * Call this to stop the last urb being unanchored from waking up any
+ * usb_wait_anchor_empty_timeout waiters. This is used in the hcd urb give-
+ * back path to delay waking up until after the completion handler has run.
+ */
+void usb_anchor_suspend_wakeups(struct usb_anchor *anchor)
+{
+	if (anchor)
+		atomic_inc(&anchor->suspend_wakeups);
+}
+EXPORT_SYMBOL_GPL(usb_anchor_suspend_wakeups);
+
+/**
+ * usb_anchor_resume_wakeups
+ * @anchor: the anchor you want to resume wakeups on
+ *
+ * Allow usb_wait_anchor_empty_timeout waiters to be woken up again, and
+ * wake up any current waiters if the anchor is empty.
+ */
+void usb_anchor_resume_wakeups(struct usb_anchor *anchor)
+{
+	if (!anchor)
+		return;
+
+	atomic_dec(&anchor->suspend_wakeups);
+	if (usb_anchor_check_wakeup(anchor))
+		wake_up(&anchor->wait);
+}
+EXPORT_SYMBOL_GPL(usb_anchor_resume_wakeups);
+
 /**
  * usb_wait_anchor_empty_timeout - wait for an anchor to be unused
  * @anchor: the anchor you want to become unused
@@ -858,7 +897,8 @@ EXPORT_SYMBOL_GPL(usb_unlink_anchored_urbs);
 int usb_wait_anchor_empty_timeout(struct usb_anchor *anchor,
 				  unsigned int timeout)
 {
-	return wait_event_timeout(anchor->wait, list_empty(&anchor->urb_list),
+	return wait_event_timeout(anchor->wait,
+				  usb_anchor_check_wakeup(anchor),
 				  msecs_to_jiffies(timeout));
 }
 EXPORT_SYMBOL_GPL(usb_wait_anchor_empty_timeout);

include/linux/usb.h

@@ -1209,6 +1209,7 @@ struct usb_anchor {
 	struct list_head urb_list;
 	wait_queue_head_t wait;
 	spinlock_t lock;
+	atomic_t suspend_wakeups;
 	unsigned int poisoned:1;
 };
 
@@ -1575,6 +1576,8 @@ extern void usb_kill_anchored_urbs(struct usb_anchor *anchor);
 extern void usb_poison_anchored_urbs(struct usb_anchor *anchor);
 extern void usb_unpoison_anchored_urbs(struct usb_anchor *anchor);
 extern void usb_unlink_anchored_urbs(struct usb_anchor *anchor);
+extern void usb_anchor_suspend_wakeups(struct usb_anchor *anchor);
+extern void usb_anchor_resume_wakeups(struct usb_anchor *anchor);
 extern void usb_anchor_urb(struct urb *urb, struct usb_anchor *anchor);
 extern void usb_unanchor_urb(struct urb *urb);
 extern int usb_wait_anchor_empty_timeout(struct usb_anchor *anchor,