linux-kernel - Re: [PATCH v2] datagram: return from __skb_recv

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Message-Id: <20180718.141559.1076017531577697883.davem@davemloft.net>
Date:   Wed, 18 Jul 2018 14:15:59 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     baoyou.xie@...il.com, willemb@...gle.com, viro@...iv.linux.org.uk,
        gregkh@...uxfoundation.org, pombredanne@...b.com,
        tklauser@...tanz.ch, matthew@...systems.ca, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] datagram: return from __skb_recv_datagram() as soon
 as possible

From: Willem de Bruijn <willemdebruijn.kernel@...il.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.