net: avoid strange behavior with skb_defer_max == 1

When user sets skb_defer_max to 1 the kick threshold is 0
(half of 1). If we increment queue length before the check
the kick will never happen, and the skb may get stranded.
This is likely harmless but can be avoided by moving the
increment after the check. This way skb_defer_max == 1
will always kick. Still a silly config to have, but
somehow that feels more correct.

While at it drop a comment which seems to be outdated
or confusing, and wrap the defer_count write with
a WRITE_ONCE() since it's read on the fast path
that avoids taking the lock.
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220518185522.2038683-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -6512,18 +6512,15 @@ nodefer:	__kfree_skb(skb);
 	if (READ_ONCE(sd->defer_count) >= defer_max)
 		goto nodefer;
 
-	/* We do not send an IPI or any signal.
-	 * Remote cpu will eventually call skb_defer_free_flush()
-	 */
 	spin_lock_irqsave(&sd->defer_lock, flags);
-	skb->next = sd->defer_list;
-	/* Paired with READ_ONCE() in skb_defer_free_flush() */
-	WRITE_ONCE(sd->defer_list, skb);
-	sd->defer_count++;
-
 	/* 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);
 
+	skb->next = sd->defer_list;
+	/* Paired with READ_ONCE() in skb_defer_free_flush() */
+	WRITE_ONCE(sd->defer_list, skb);
 	spin_unlock_irqrestore(&sd->defer_lock, flags);
 
 	/* Make sure to trigger NET_RX_SOFTIRQ on the remote CPU