linux-kernel - Re: [PATCH RFC] IPv6: Avoid taking write lock for /proc/net/ipv6

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Message-Id: <20111230.170905.1348056803377924696.davem@davemloft.net>
Date:	Fri, 30 Dec 2011 17:09:05 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	joshhunt00@...il.com
Cc:	kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] IPv6: Avoid taking write lock for
 /proc/net/ipv6_route

From: Josh Hunt <joshhunt00@...il.com>
Date: Wed, 28 Dec 2011 17:23:07 -0600

> lock_stat shows taking the write lock is causing the slowdown. Using
> this info I decided to write a version of fib6_clean_all() which
> replaces write_lock_bh(&table->tb6_lock) with
> read_lock_bh(&table->tb6_lock). With this new function I see the same
> results as with my rtnetlink iperf test. I guess my question is what
> am I missing? Is there a reason you need to take the write lock when
> reading the route table to display to proc?

You're not missing anything, it's just an oversight or laziness. :-)

I've applied your patch thanks.

Longer term we should make the ipv6 tree traversals RCU safe just
like net/ipv4/fib_trie.c is.  Then we can do away with even the
read locks for read-only traversals.

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