netdev - Re: [PATCH net] net: neighbour: add neighbour dead check for neigh_timer

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Message-Id: <20131204.013638.2211412920346095879.davem@davemloft.net>
Date:	Wed, 04 Dec 2013 01:36:38 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	dingtianhong@...wei.com
Cc:	gaofeng@...fujitsu.com, yoshfuji@...ux-ipv6.org, joe@...ches.com,
	vfalico@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] net: neighbour: add neighbour dead check for
 neigh_timer_handler()

From: Ding Tianhong <dingtianhong@...wei.com>
Date: Wed, 4 Dec 2013 14:19:06 +0800

> Yes, you are right, but when the timer is running and prior to get
> the neigh->lock, the refcnt could be dec to 0, you could not stop it
> by existing mechanism.
> 
> the refcnt of neighbour could only be inc by these actions:
> 
> 1.create neighbour, the refcnt will be set to 1.
> 2.add timer, the refcnt++.
> 3.neigh_lookup, if found the neigh, refcnt++.

The refcnt cannot dec to 0 if the timer is running, that's why
the last action of the timer handler is neigh_release().

The timer handler always holds a reference to the neighbour entry for
which it is running, that is why it must always release it.

If it can dec to 0, then this neigh_release() call in the timer
handler is illegal.  But it is not.
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