Re: [PATCH v2] datagram: return from __skb_recv_datagram() as soon as possible

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Mon, 16 Jul 2018 15:17:54 -0700

If the above occurs, that implies that the queue is not empty so the
next iteration of the loop in __skb_recv_datagram should return
the oldest packet on the queue.

Isn't it possible, with two threads pulling from the socket in
parallel, for one of them to be constantly unable to pass that
test:

	if (sk->sk_receive_queue.prev != skb)
		goto out;

because the other one empties the queue too quickly every time?

We sample 'last' with the queue lock held, but the above test is done
without that lock.