netdev - skb_linearize

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Message-ID: <20120916091747.GA23775@redhat.com>
Date:	Sun, 16 Sep 2012 12:17:47 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	netdev@...r.kernel.org, Herbert Xu <herbert@...dor.hengli.com.au>
Subject: skb_linearize

I notice that dev_hard_start_xmit might invoke
__skb_linearize e.g. if device does not support NETIF_F_SG.

This in turn onvokes __pskb_pull_tail, and
documentation of __pskb_pull_tail says:
  &sk_buff MUST have reference count of 1.

I am guessing 'reference count' means users in this context, right?
IIUC this is because it modifies skb in a way that
isn't safe if anyone else is looking at the skb.

However, I don't see what guarantees that reference
count is 1 when dev_hard_start_xmit invokes
linearize. In particular it calls dev_queue_xmit_nit
which could queue packets on a network tap.

Could someone help me understand please?

Thanks!

-- 
MST
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