netdev - Re: [PATCH] netlink: enable skb header refcounting before sending first broadcast

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Date:	Mon, 13 Jul 2015 10:22:34 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] netlink: enable skb header refcounting before sending
 first broadcast

On Mon, 2015-07-13 at 16:10 +0800, Herbert Xu wrote:
> On Mon, Jul 13, 2015 at 10:05:42AM +0200, Eric Dumazet wrote:
> >
> > Herbert, UDP peek support is very buggy anyway, because of deferred
> > checksums
> > 
> > __skb_checksum_complete() will happily manipulate csum, ip_summed,
> > csum_complete_sw & csum_valid
> > 
> > Ideally, peek should never touch skb (but skb->users)
> 
> I think UDP should be OK because the main creator of shared skbs
> is af_packet and in that cast the IP stack will clone the skb upon
> entry.  AFAIK there aren't any entities doing the shared skb trick
> within the IP stack.
> 
> IOW the UDP stack does not have to worry about share skbs, unlike
> netlink.

It should worry, in case multiple threads are using MSG_PEEK on same udp
socket ;)

Problem here is not the producer (might be unicast packets btw),
but multiple 'consumers'

It turns out your patch would also solve this problem.


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