[PATCH] USB: fix a thread-exit problem at module unload

This patch fixes a thread-exit problem when the usb-storage module is
unloaded with a preemptable kernel.  Please refer to the comments in the
code for more detail.

drivers/usb/storage/usb.c

@@ -417,10 +417,21 @@ static int usb_stor_control_thread(void * __us)
 		scsi_unlock(host);
 	} /* for (;;) */
 
-	/* notify the exit routine that we're actually exiting now */
-	complete(&(us->notify));
-
-	return 0;
+	/* notify the exit routine that we're actually exiting now 
+	 *
+	 * complete()/wait_for_completion() is similar to up()/down(),
+	 * except that complete() is safe in the case where the structure
+	 * is getting deleted in a parallel mode of execution (i.e. just
+	 * after the down() -- that's necessary for the thread-shutdown
+	 * case.
+	 *
+	 * complete_and_exit() goes even further than this -- it is safe in
+	 * the case that the thread of the caller is going away (not just
+	 * the structure) -- this is necessary for the module-remove case.
+	 * This is important in preemption kernels, which transfer the flow
+	 * of execution immediately upon a complete().
+	 */
+	complete_and_exit(&(us->notify), 0);
 }	
 
 /***********************************************************************