netdev - Re: [PATCH 0/3] net: time stamping fixes

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Date:	Wed, 19 Oct 2011 13:50:12 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	eric.dumazet@...il.com
Subject: Re: [PATCH 0/3] net: time stamping fixes

On Wed, Oct 19, 2011 at 07:15:36AM +0200, Johannes Berg wrote:
> The only thing I'm not completely sure about is whether or not it is
> permissible to sock_hold() at that point. I'm probably just missing
> something, but: if sk_free() was called before hard_start_xmit() which
> will call skb_clone_tx_timestamp(), can we really call sock_hold()?
> 
> The reason I ask is that sock_wfree() doesn't check sk_refcnt, so if it
> is possible for sk_free() to have been called before hard_start_xmit(),
> maybe because the packet was stuck on the qdisc for a while, the socket
> won't be released (sk_free checks sk_wmem_alloc) but the sk_wfree() when
> the original skb is freed will actually free the socket, invalidating
> the clone's sk pointer *even though* we called sock_hold() right after
> making the clone.
> 
> So what guarantees that sk_refcnt is still non-zero when we make the
> clone?

In the non-qdisc path, the kernel is in a send() call, so the initial
reference taken in socket() is held.

I really don't know the qdisc code, whether it is somehow holding the
skb->sk indirectly or not.

Eric? David?

> Alternatively, should sock_wfree() check sk_refcnt?

That would presumably spoil the performance enhancement gained in
commit 2b85a34e.
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