async: make sure independent async domains can't accidentally entangle

The problem occurs when async_synchronize_full_domain() is called when
the async_pending list is not empty.  This will cause lowest_running()
to return the cookie of the first entry on the async_pending list, which
might be nothing at all to do with the domain being asked for and thus
cause the domain synchronization to wait for an unrelated domain.   This
can cause a deadlock if domain synchronization is used from one domain
to wait for another.

Fix by running over the async_pending list to see if any pending items
actually belong to our domain (and return their cookies if they do).
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/async.c

@@ -92,19 +92,23 @@ extern int initcall_debug;
 static async_cookie_t  __lowest_in_progress(struct list_head *running)
 {
 	struct async_entry *entry;
+	async_cookie_t ret = next_cookie; /* begin with "infinity" value */
+
 	if (!list_empty(running)) {
 		entry = list_first_entry(running,
 			struct async_entry, list);
-		return entry->cookie;
-	} else if (!list_empty(&async_pending)) {
-		entry = list_first_entry(&async_pending,
-			struct async_entry, list);
-		return entry->cookie;
-	} else {
-		/* nothing in progress... next_cookie is "infinity" */
-		return next_cookie;
+		ret = entry->cookie;
 	}
 
+	if (!list_empty(&async_pending)) {
+		list_for_each_entry(entry, &async_pending, list)
+			if (entry->running == running) {
+				ret = entry->cookie;
+				break;
+			}
+	}
+
+	return ret;
 }
 
 static async_cookie_t  lowest_in_progress(struct list_head *running)