pktgen: make sure that pktgen_thread_worker has been executed

The following courruption can happen during pktgen stop:
list_del corruption. prev->next should be ffff81007e8a5e70, but was 6b6b6b6b6b6b6b6b
kernel BUG at lib/list_debug.c:67!
      :pktgen:pktgen_thread_worker+0x374/0x10b0
      ? autoremove_wake_function+0x0/0x40
      ? _spin_unlock_irqrestore+0x42/0x80
      ? :pktgen:pktgen_thread_worker+0x0/0x10b0
      kthread+0x4d/0x80
      child_rip+0xa/0x12
      ? restore_args+0x0/0x30
      ? kthread+0x0/0x80
      ? child_rip+0x0/0x12
RIP  list_del+0x48/0x70

The problem is that pktgen_thread_worker can not be executed if kthread_stop
has been called too early. Insert a completion on the normal initialization
path to make sure that pktgen_thread_worker will gain the control for sure.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Acked-by: NAlexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -390,6 +390,7 @@ struct pktgen_thread {
 	int cpu;
 
 	wait_queue_head_t queue;
+	struct completion start_done;
 };
 
 #define REMOVE 1
@@ -3414,6 +3415,7 @@ static int pktgen_thread_worker(void *arg)
 	BUG_ON(smp_processor_id() != cpu);
 
 	init_waitqueue_head(&t->queue);
+	complete(&t->start_done);
 
 	pr_debug("pktgen: starting pktgen/%d:  pid=%d\n", cpu, task_pid_nr(current));
 
@@ -3615,6 +3617,7 @@ static int __init pktgen_create_thread(int cpu)
 	INIT_LIST_HEAD(&t->if_list);
 
 	list_add_tail(&t->th_list, &pktgen_threads);
+	init_completion(&t->start_done);
 
 	p = kthread_create(pktgen_thread_worker, t, "kpktgend_%d", cpu);
 	if (IS_ERR(p)) {
@@ -3639,6 +3642,7 @@ static int __init pktgen_create_thread(int cpu)
 	}
 
 	wake_up_process(p);
+	wait_for_completion(&t->start_done);
 
 	return 0;
 }