pktgen: use cpus_read_lock() in pg_net_init()

I have seen the WARN_ON(smp_processor_id() != cpu) firing
in pktgen_thread_worker() during tests.

We must use cpus_read_lock()/cpus_read_unlock()
around the for_each_online_cpu(cpu) loop.

While we are at it use WARN_ON_ONCE() to avoid a possible syslog flood.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240821175339.1191779-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/pktgen.c

@@ -3654,7 +3654,7 @@ static int pktgen_thread_worker(void *arg)
 	struct pktgen_dev *pkt_dev = NULL;
 	int cpu = t->cpu;
 
-	WARN_ON(smp_processor_id() != cpu);
+	WARN_ON_ONCE(smp_processor_id() != cpu);
 
 	init_waitqueue_head(&t->queue);
 	complete(&t->start_done);
@@ -3989,6 +3989,7 @@ static int __net_init pg_net_init(struct net *net)
 		goto remove;
 	}
 
+	cpus_read_lock();
 	for_each_online_cpu(cpu) {
 		int err;
 
@@ -3997,6 +3998,7 @@ static int __net_init pg_net_init(struct net *net)
 			pr_warn("Cannot create thread for cpu %d (%d)\n",
 				   cpu, err);
 	}
+	cpus_read_unlock();
 
 	if (list_empty(&pn->pktgen_threads)) {
 		pr_err("Initialization failed for all threads\n");