SUNRPC: We must not use list_for_each_entry_safe() in rpc_wake_up()

The problem is that for the case of priority queues, we
have to assume that __rpc_remove_wait_queue_priority will move new
elements from the tk_wait.links lists into the queue->tasks[] list.
We therefore cannot use list_for_each_entry_safe() on queue->tasks[],
since that will skip these new tasks that __rpc_remove_wait_queue_priority
is adding.

Without this fix, rpc_wake_up and rpc_wake_up_status will both fail
to wake up all functions on priority wait queues, which can result
in some nasty hangs.
Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org

net/sunrpc/sched.c

@@ -534,14 +534,18 @@ EXPORT_SYMBOL_GPL(rpc_wake_up_next);
  */
 void rpc_wake_up(struct rpc_wait_queue *queue)
 {
-	struct rpc_task *task, *next;
 	struct list_head *head;
 
 	spin_lock_bh(&queue->lock);
 	head = &queue->tasks[queue->maxpriority];
 	for (;;) {
-		list_for_each_entry_safe(task, next, head, u.tk_wait.list)
+		while (!list_empty(head)) {
+			struct rpc_task *task;
+			task = list_first_entry(head,
+					struct rpc_task,
+					u.tk_wait.list);
 			rpc_wake_up_task_queue_locked(queue, task);
+		}
 		if (head == &queue->tasks[0])
 			break;
 		head--;
@@ -559,13 +563,16 @@ EXPORT_SYMBOL_GPL(rpc_wake_up);
  */
 void rpc_wake_up_status(struct rpc_wait_queue *queue, int status)
 {
-	struct rpc_task *task, *next;
 	struct list_head *head;
 
 	spin_lock_bh(&queue->lock);
 	head = &queue->tasks[queue->maxpriority];
 	for (;;) {
-		list_for_each_entry_safe(task, next, head, u.tk_wait.list) {
+		while (!list_empty(head)) {
+			struct rpc_task *task;
+			task = list_first_entry(head,
+					struct rpc_task,
+					u.tk_wait.list);
 			task->tk_status = status;
 			rpc_wake_up_task_queue_locked(queue, task);
 		}