SUNRPC: When changing the queue priority, ensure that we change the owner

This fixes a livelock in the xprt->sending queue where we end up never
making progress on lower priority tasks because sleep_on_priority()
keeps adding new tasks with the same owner to the head of the queue,
and priority bumps mean that we keep resetting the queue->owner to
whatever task is at the head of the queue.

Regression introduced by commit c05eecf636101dd4347b2d8fa457626bf0088e0a
(SUNRPC: Don't allow low priority tasks to pre-empt higher priority ones).

Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index bfa31714581f52e27f51c312aa0288a9cb4fd411..fb20f25ddec9b70c805ad65d675441d02b62055b 100644 (file)
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -98,9 +98,25 @@ __rpc_add_timer(struct rpc_wait_queue *queue, struct rpc_task *task)
        list_add(&task->u.tk_wait.timer_list, &queue->timer_list.list);
 }
 
+static void rpc_rotate_queue_owner(struct rpc_wait_queue *queue)
+{
+       struct list_head *q = &queue->tasks[queue->priority];
+       struct rpc_task *task;
+
+       if (!list_empty(q)) {
+               task = list_first_entry(q, struct rpc_task, u.tk_wait.list);
+               if (task->tk_owner == queue->owner)
+                       list_move_tail(&task->u.tk_wait.list, q);
+       }
+}
+
 static void rpc_set_waitqueue_priority(struct rpc_wait_queue *queue, int priority)
 {
-       queue->priority = priority;
+       if (queue->priority != priority) {
+               /* Fairness: rotate the list when changing priority */
+               rpc_rotate_queue_owner(queue);
+               queue->priority = priority;
+       }
 }
 
 static void rpc_set_waitqueue_owner(struct rpc_wait_queue *queue, pid_t pid)