nfs4: fix potential race with rapid nfs_callback_up/down cycle

If the nfsv4 callback thread is rapidly brought up and down, it's
possible that nfs_callback_svc might never get a chance to run. If
this happens, the cleanup at thread exit might never occur, throwing
the refcounting off and nfs_callback_info in an incorrect state.

Move the clean functions into nfs_callback_down. Also change the
nfs_callback_info struct to track the svc_rqst rather than svc_serv
since we need to know that to call svc_exit_thread.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/callback.c

@@ -27,7 +27,7 @@
 
 struct nfs_callback_data {
 	unsigned int users;
-	struct svc_serv *serv;
+	struct svc_rqst *rqst;
 	struct task_struct *task;
 };
 
@@ -91,18 +91,15 @@ nfs_callback_svc(void *vrqstp)
 		svc_process(rqstp);
 	}
 	unlock_kernel();
-	nfs_callback_info.task = NULL;
-	svc_exit_thread(rqstp);
 	return 0;
 }
 
 /*
- * Bring up the server process if it is not already up.
+ * Bring up the callback thread if it is not already up.
  */
 int nfs_callback_up(void)
 {
 	struct svc_serv *serv = NULL;
-	struct svc_rqst *rqstp;
 	int ret = 0;
 
 	mutex_lock(&nfs_callback_mutex);
@@ -120,22 +117,23 @@ int nfs_callback_up(void)
 	nfs_callback_tcpport = ret;
 	dprintk("Callback port = 0x%x\n", nfs_callback_tcpport);
 
-	rqstp = svc_prepare_thread(serv, &serv->sv_pools[0]);
-	if (IS_ERR(rqstp)) {
-		ret = PTR_ERR(rqstp);
+	nfs_callback_info.rqst = svc_prepare_thread(serv, &serv->sv_pools[0]);
+	if (IS_ERR(nfs_callback_info.rqst)) {
+		ret = PTR_ERR(nfs_callback_info.rqst);
+		nfs_callback_info.rqst = NULL;
 		goto out_err;
 	}
 
 	svc_sock_update_bufs(serv);
-	nfs_callback_info.serv = serv;
 
-	nfs_callback_info.task = kthread_run(nfs_callback_svc, rqstp,
+	nfs_callback_info.task = kthread_run(nfs_callback_svc,
+					     nfs_callback_info.rqst,
 					     "nfsv4-svc");
 	if (IS_ERR(nfs_callback_info.task)) {
 		ret = PTR_ERR(nfs_callback_info.task);
-		nfs_callback_info.serv = NULL;
+		svc_exit_thread(nfs_callback_info.rqst);
+		nfs_callback_info.rqst = NULL;
 		nfs_callback_info.task = NULL;
-		svc_exit_thread(rqstp);
 		goto out_err;
 	}
 out:
@@ -157,14 +155,18 @@ int nfs_callback_up(void)
 }
 
 /*
- * Kill the server process if it is not already down.
+ * Kill the callback thread if it's no longer being used.
  */
 void nfs_callback_down(void)
 {
 	mutex_lock(&nfs_callback_mutex);
 	nfs_callback_info.users--;
-	if (nfs_callback_info.users == 0 && nfs_callback_info.task != NULL)
+	if (nfs_callback_info.users == 0 && nfs_callback_info.task != NULL) {
 		kthread_stop(nfs_callback_info.task);
+		svc_exit_thread(nfs_callback_info.rqst);
+		nfs_callback_info.rqst = NULL;
+		nfs_callback_info.task = NULL;
+	}
 	mutex_unlock(&nfs_callback_mutex);
 }