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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH v2] datagram: return from __skb_recv_datagram() as soon
 as possible
Date: Wed, 18 Jul 2018 14:15:59 +0900 (KST)
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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.