netdev - Re: [PATCH] net: remove superfluous call to synchronize

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Date:	Fri, 17 Apr 2009 04:56:31 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	paulmck@...ux.vnet.ibm.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: remove superfluous call to synchronize_net()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Thu, 16 Apr 2009 07:40:23 +0200

> [PATCH] net: remove superfluous call to synchronize_net()
> 
> inet_register_protosw() function is responsible for adding a new
> inet protocol into a global table (inetsw[]) that is used with RCU rules.
> 
> As soon as the store of the pointer is done, other cpus might see
> this new protocol in inetsw[], so we have to make sure new protocol
> is ready for use. All pending memory updates should thus be committed
> to memory before setting the pointer.
> This is correctly done using rcu_assign_pointer()
> 
> synchronize_net() is typically used at unregister time, after
> unsetting the pointer, to make sure no other cpu is still using
> the object we want to dismantle. Using it at register time
> is only adding an artificial delay that could hide a real bug,
> and this bug could popup if/when synchronize_rcu() can proceed
> faster than now.
> 
> This saves about 13 ms on boot time on a HZ=1000 8 cpus machine  ;) 
> (4 calls to inet_register_protosw(), and about 3200 us per call)
> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

I think this change is fine, so I'm adding it to net-next-2.6

If you guys want to continue discussing the merits of putting
comments in every spot where we lack a RCU sync call, that's
your call. :-)

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