netdev - Re: [PATCH v2] net IPv6 : Fix broken IPv6 routing table after loopback down-up

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Message-ID: <1364496577.15753.51.camel@edumazet-glaptop>
Date:	Thu, 28 Mar 2013 11:49:37 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Balakumaran Kannan <kumaran.4353@...il.com>,
	yoshfuji@...ux-ipv6.org, davem@...emloft.net,
	Patrick McHardy <kaber@...sh.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>, jmorris@...ei.org,
	Balakumaran.Kannan@...sony.com, maruthi.thotad@...sony.com,
	netdev@...r.kernel.org, jamshed.a@...sony.com,
	amit.agarwal@...sony.com, takuzo.ohara@...sony.com,
	aaditya.kumar@...sony.com
Subject: Re: [PATCH v2] net IPv6 : Fix broken IPv6 routing table after
 loopback down-up

On Thu, 2013-03-28 at 11:23 -0700, Paul E. McKenney wrote:

> 
> Agreed, there is no point in using RCU when it is not needed.
> 
> That said...
> 
> Since v3.1, in CONFIG_PREEMPT=y kernels, rcu_read_lock() does not
> disable preemption.  In CONFIG_PREEMPT=n kernels, rcu_read_lock() does
> disable preemption, but to no effect because preemption is already
> disabled anyway.
> 
> The net effect is that rcu_read_lock() has no effect on preemption.
> 

Good point, but this patch might be a stable candidate.

Rule of thumb for networking is 

1) Control path : RTNL mutex

2) Data path : RTNL cant be taken (from softirq), so use RCU if
possible.

Thanks !


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