[PATCH] p54usb: fix usb_kill_urb hang with slub_debug=P

[Christian Lamparter <chunkeey@web.de>]

This patch fixes a problem identified by Johannes Berg.

It looks like this is a classic case of "use-after-freed". 
A module which should reproduce the problem on
any other USB device can be found right here:
http://kerneltrap.org/mailarchive/linux-usb/2008/12/4/4317064

Tested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
CC: stable@vger.kernel.org
---
diff --git a/drivers/net/wireless/p54/p54usb.c b/drivers/net/wireless/p54/p54usb.c
index 3444f52..0b9a922 100644
--- a/drivers/net/wireless/p54/p54usb.c
+++ b/drivers/net/wireless/p54/p54usb.c
@@ -209,7 +209,14 @@ static void p54u_free_urbs(struct ieee80211_hw *dev)
 		if (!info->urb)
 			continue;
 
+		/*
+		 * usb_get_urb and usb_free_urb are part of a temporary
+		 * workaround. Otherwise we get a pretty bad freeze,
+		 * if SLUB's poisoning debug option is enabled.
+		 */
+		usb_get_urb(info->urb);
 		usb_kill_urb(info->urb);
+		usb_free_urb(info->urb);
 		kfree_skb(skb);
 	}
 }