net: no longer assume RTNL is held in flush_all_backlogs()

flush_all_backlogs() uses per-cpu and static data to hold its
temporary data, on the assumption it is called under RTNL
protection.

Following patch in the series will break this assumption.

Use instead a dynamically allocated piece of memory.

In the unlikely case the allocation fails,
use a boot-time allocated memory.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jesse Brandeburg <jbrandeburg@cloudflare.com>
Link: https://patch.msgid.link/20250114205531.967841-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -6013,8 +6013,6 @@ void netif_receive_skb_list(struct list_head *head)
 }
 EXPORT_SYMBOL(netif_receive_skb_list);
 
-static DEFINE_PER_CPU(struct work_struct, flush_works);
-
 /* Network device is going away, flush any packets still pending */
 static void flush_backlog(struct work_struct *work)
 {
@@ -6071,36 +6069,54 @@ static bool flush_required(int cpu)
 	return true;
 }
 
+struct flush_backlogs {
+	cpumask_t		flush_cpus;
+	struct work_struct	w[];
+};
+
+static struct flush_backlogs *flush_backlogs_alloc(void)
+{
+	return kmalloc(struct_size_t(struct flush_backlogs, w, nr_cpu_ids),
+		       GFP_KERNEL);
+}
+
+static struct flush_backlogs *flush_backlogs_fallback;
+static DEFINE_MUTEX(flush_backlogs_mutex);
+
 static void flush_all_backlogs(void)
 {
-	static cpumask_t flush_cpus;
+	struct flush_backlogs *ptr = flush_backlogs_alloc();
 	unsigned int cpu;
 
-	/* since we are under rtnl lock protection we can use static data
-	 * for the cpumask and avoid allocating on stack the possibly
-	 * large mask
-	 */
-	ASSERT_RTNL();
+	if (!ptr) {
+		mutex_lock(&flush_backlogs_mutex);
+		ptr = flush_backlogs_fallback;
+	}
+	cpumask_clear(&ptr->flush_cpus);
 
 	cpus_read_lock();
 
-	cpumask_clear(&flush_cpus);
 	for_each_online_cpu(cpu) {
 		if (flush_required(cpu)) {
-			queue_work_on(cpu, system_highpri_wq,
-				      per_cpu_ptr(&flush_works, cpu));
-			cpumask_set_cpu(cpu, &flush_cpus);
+			INIT_WORK(&ptr->w[cpu], flush_backlog);
+			queue_work_on(cpu, system_highpri_wq, &ptr->w[cpu]);
+			__cpumask_set_cpu(cpu, &ptr->flush_cpus);
 		}
 	}
 
 	/* we can have in flight packet[s] on the cpus we are not flushing,
 	 * synchronize_net() in unregister_netdevice_many() will take care of
-	 * them
+	 * them.
 	 */
-	for_each_cpu(cpu, &flush_cpus)
-		flush_work(per_cpu_ptr(&flush_works, cpu));
+	for_each_cpu(cpu, &ptr->flush_cpus)
+		flush_work(&ptr->w[cpu]);
 
 	cpus_read_unlock();
+
+	if (ptr != flush_backlogs_fallback)
+		kfree(ptr);
+	else
+		mutex_unlock(&flush_backlogs_mutex);
 }
 
 static void net_rps_send_ipi(struct softnet_data *remsd)
@@ -12313,12 +12329,13 @@ static int __init net_dev_init(void)
 	 *	Initialise the packet receive queues.
 	 */
 
+	flush_backlogs_fallback = flush_backlogs_alloc();
+	if (!flush_backlogs_fallback)
+		goto out;
+
 	for_each_possible_cpu(i) {
-		struct work_struct *flush = per_cpu_ptr(&flush_works, i);
 		struct softnet_data *sd = &per_cpu(softnet_data, i);
 
-		INIT_WORK(flush, flush_backlog);
-
 		skb_queue_head_init(&sd->input_pkt_queue);
 		skb_queue_head_init(&sd->process_queue);
 #ifdef CONFIG_XFRM_OFFLOAD