From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] datagram: Extend the datagram queue MSG_PEEK-ing
Date: Tue, 21 Feb 2012 00:01:01 -0500 (EST)
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From: Pavel Emelyanov <xemul@parallels.com>
Date: Mon, 20 Feb 2012 16:17:11 +0400

> I was thinking about another option of doing the same, how about introducing
> the peek offset member on sock (get/set via sockopt) which works like
> 
> * if == -1, then peek works as before
> * if >= 0, then each peek/recvmsg will increase/decrease the value so that
>   the next peek peeks next data
> 
> It's questionable what to do if the peek_off points into the middle of a
> datagram however. Here's an example of how this looks for datagram sockets
> (tested on pf_unix), for stream this requires more patching.
> 
> What do you think? Does it make sense to go on with this making other 
> ->recvmsg handlers support peeking offset?

I think you need to add some kind of locking to this for sanity, for
example right now you're doing test/decide/store decisions over
sk_peek_off inside of __skb_recv_datagram() without the socket lock,
yet another thread can set the SO_PEEK_OFF in parallel.

The locking decision will probably be worse for stream sockets.