netdev - Re: Question about synchronize_net() in AF

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Date:	Tue, 09 Sep 2014 15:00:00 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	martin@...tingkelly.com
Cc:	netdev@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	stephen@...workplumber.org
Subject: Re: Question about synchronize_net() in AF_PACKET close()

From: Martin Kelly <martin@...tingkelly.com>
Date: Tue, 9 Sep 2014 14:44:56 -0700

> In net/packet/af_packet.c, I noticed the following few lines within
> packet_release:
> 
>          synchronize_net();
>          /*
>           *      Now the socket is dead. No more input will appear.
>           */
>          sock_orphan(sk);
>          sock->sk = NULL;
> 
> From testing and code analysis, I have found that it appears to be safe
> to move sock_orphan and sock->sk = NULL before the synchronize_net()
> call like so:
> 
>          /*
>           *      Now the socket is dead. No more input will appear.
>           */
>          sock_orphan(sk);
>          sock->sk = NULL;
>          synchronize_net();
> 
> Could some RCU and/or networking experts chime in about whether this a
> safe operation? For all I know, there is some deep, fundamental reason
> why those lines are in the order they are. On the other hand, perhaps
> there is not.

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.

You probably want to reference commit:

commit 808f5114a9206fee855117d416440e1071ab375c
Author: stephen hemminger <shemminger@...tta.com>
Date:   Mon Feb 22 07:57:18 2010 +0000

    packet: convert socket list to RCU (v3)

which put the synchronize_net() there in the first place.
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