netdev - Re: [PATCH] fix NULL pointer + success return in route lookup path

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Message-ID: <20090622112956.GA22313@ms2.inr.ac.ru>
Date:	Mon, 22 Jun 2009 15:29:56 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	mbizon@...ebox.fr, dada1@...mosbay.com, pekkas@...core.fi,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH] fix NULL pointer + success return in route lookup path

On Mon, Jun 22, 2009 at 11:00:23AM +0000, Jarek Poplawski wrote:
> One little doubt would be RCU: if it's currently used in rt_free, and
> some code depends on it, there would be a change: rt could be freed
> just after atomic dec, without waiting for RCU yet.

Is this bad 'could', or "would be good 'could'"? :-)

Such route always goes through RCU: nobody can destroy it,
destruction happens only if route is on gc list or when RCU callback
is called and refcnt is already 0.

BTW RCU on such routes is redundant, they never were in hash table.
This means that rt_free() could be safely replaced with straight dst_free()
in this place.

>					 The second one is
> about timing: freeing this always from a workqueue could probably
> make a problem if softirqs are often disabled.

Do you mean dst_gc_task?

Yes, it could be optimized out, route can be destroyed when
refcnt hits zero, but this will require checking refcnt in dst_release().
Old days it was well visible both on code size and performance.
Probably, nowadays it is not a problem anymore.

Alexey
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