net: make softnet_data.defer_count an atomic

This is preparation work to remove the softnet_data.defer_lock,
as it is contended on hosts with large number of cores.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250928084934.3266948-2-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: edumazet

include/linux/netdevice.h

@@ -3538,7 +3538,7 @@ struct softnet_data {
 
 	/* Another possibly contended cache line */
 	spinlock_t		defer_lock ____cacheline_aligned_in_smp;
-	int			defer_count;
+	atomic_t		defer_count;
 	int			defer_ipi_scheduled;
 	struct sk_buff		*defer_list;
 	call_single_data_t	defer_csd;

net/core/dev.c

@@ -6726,7 +6726,7 @@ static void skb_defer_free_flush(struct softnet_data *sd)
 	spin_lock(&sd->defer_lock);
 	skb = sd->defer_list;
 	sd->defer_list = NULL;
-	sd->defer_count = 0;
+	atomic_set(&sd->defer_count, 0);
 	spin_unlock(&sd->defer_lock);
 
 	while (skb != NULL) {

net/core/skbuff.c

@@ -7202,14 +7202,12 @@ nodefer:	kfree_skb_napi_cache(skb);
 
 	sd = &per_cpu(softnet_data, cpu);
 	defer_max = READ_ONCE(net_hotdata.sysctl_skb_defer_max);
-	if (READ_ONCE(sd->defer_count) >= defer_max)
+	if (atomic_read(&sd->defer_count) >= defer_max)
 		goto nodefer;
 
 	spin_lock_bh(&sd->defer_lock);
 	/* Send an IPI every time queue reaches half capacity. */
-	kick = sd->defer_count == (defer_max >> 1);
-	/* Paired with the READ_ONCE() few lines above */
-	WRITE_ONCE(sd->defer_count, sd->defer_count + 1);
+	kick = (atomic_inc_return(&sd->defer_count) - 1) == (defer_max >> 1);
 
 	skb->next = sd->defer_list;
 	/* Paired with READ_ONCE() in skb_defer_free_flush() */