netdev - Re: Question about synchronize_net() in AF

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Message-ID: <CAN8CM3xEnQXVgrLpV0rfAJoZ75v6Xv64pAGJ-9SAPACAT8WJ_w@mail.gmail.com>
Date:	Wed, 10 Sep 2014 14:37:41 -0700
From:	Martin Kelly <martin@...tingkelly.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: Question about synchronize_net() in AF_PACKET close()

> The synchronize_net() is also there to protect against the prot hook
> which can run asynchronously from the core packet input path on any
> cpu.
>

Yes, understood. What I'm not clear about is whether it is safe to do
the following:

unregister_prot_hook(sk, false);
sock_orphan(sk);
sock->sk = NULL;
call_rcu(...);
close socket, return to userspace

instead of

unregister_prot_hook(sk, false);
synchronize_net();
sock_orphan(sk);
sock->sk = NULL;
close socket, return to userspace

If you don't call synchronize_net() immediately, then other readers
could see the protocol hook in the protocol list and try to use it.
They could call into prot_hook.func. However, it appears that  such
functions ( e.g. packet_rcv() ) touch the socket buffer but not the
socket itself, so orphaning the socket before all RCUs have been
processed is safe. In addition, no new packets will come in after
packet_release() and touch the socket because the socket fd will be
removed from the process fd list.

>From my testing, I'm not seeing any obvious issues, but I could be
missing something. Is orphaning the socket before all RCUs have
finished unsafe?
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