netdev - Re: [PATCH] ipv4: fix the rcu race between free_fib_info and ip_route_output

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Date:	Wed, 23 May 2012 06:37:09 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	yanmin_zhang@...ux.intel.com, kunx.jiang@...el.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipv4: fix the rcu race between free_fib_info and
 ip_route_output_slow

On Tue, 2012-05-22 at 23:23 -0400, David Miller wrote:
> From: Yanmin Zhang <yanmin_zhang@...ux.intel.com>
> Date: Wed, 23 May 2012 11:02:03 +0800
> 
> > 1) Why does free_fib_info call call_rcu instead of releasing fi directly?
> > I assume other cpu might be accessing it. nexthop_nh->nh_dev is in fi.
> > If other cpu are accessing it, here resetting to NULL would cause other
> > cpu panic.
> 
> Because fib trie lookups are done with RCU locking, therefore we must
> use RCU freeing to release the object.
> 
> What I was trying to impart to you is that removing the NULL
> assignment is wrong and that an alternative fix is warranted (hint:
> consider moving something into the RCU release).
> --

Its more than that I'm afraid.

fi->fib_dev (aka fib_nh[0].nh_dev) need full RCU protection.

Also the "fib_info_cnt--;" must stay in free_fib_info() (so that it is
protected by RTNL)



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