SUNRPC: Fix a server shutdown leak

Fix a race where kthread_stop() may prevent the threadfn from ever getting
called.  If that happens the svc_rqst will not be cleaned up.

Fixes: ed6473dd ("NFSv4: Fix callback server shutdown")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc.c

@@ -798,6 +798,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 static int
 svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 {
+	struct svc_rqst	*rqstp;
 	struct task_struct *task;
 	unsigned int state = serv->sv_nrthreads-1;
 
@@ -806,7 +807,10 @@ svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
 		task = choose_victim(serv, pool, &state);
 		if (task == NULL)
 			break;
-		kthread_stop(task);
+		rqstp = kthread_data(task);
+		/* Did we lose a race to svo_function threadfn? */
+		if (kthread_stop(task) == -EINTR)
+			svc_exit_thread(rqstp);
 		nrservs++;
 	} while (nrservs < 0);
 	return 0;