SUNRPC: Add barriers to ensure read ordering in rpc_wake_up_task_queue_locked

We need to be careful when testing task->tk_waitqueue in
rpc_wake_up_task_queue_locked, because it can be changed while we
are holding the queue->lock.
By adding appropriate memory barriers, we can ensure that it is safe to
test task->tk_waitqueue for equality if the RPC_TASK_QUEUED bit is set.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index fb20f25..f8529fc 100644 (file)
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -180,6 +180,8 @@ static void __rpc_add_wait_queue(struct rpc_wait_queue *queue,
                list_add_tail(&task->u.tk_wait.list, &queue->tasks[0]);
        task->tk_waitqueue = queue;
        queue->qlen++;
+       /* barrier matches the read in rpc_wake_up_task_queue_locked() */
+       smp_wmb();
        rpc_set_queued(task);
 
        dprintk("RPC: %5u added to queue %p \"%s\"\n",
@@ -430,8 +432,11 @@ static void __rpc_do_wake_up_task(struct rpc_wait_queue *queue, struct rpc_task
  */
 static void rpc_wake_up_task_queue_locked(struct rpc_wait_queue *queue, struct rpc_task *task)
 {
-       if (RPC_IS_QUEUED(task) && task->tk_waitqueue == queue)
-               __rpc_do_wake_up_task(queue, task);
+       if (RPC_IS_QUEUED(task)) {
+               smp_rmb();
+               if (task->tk_waitqueue == queue)
+                       __rpc_do_wake_up_task(queue, task);
+       }
 }
 
 /*