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

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Message-ID: <20090417192551.GA15538@linux.vnet.ibm.com>
Date:	Fri, 17 Apr 2009 12:25:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: remove superfluous call to synchronize_net()

On Fri, Apr 17, 2009 at 04:56:31AM -0700, David Miller wrote:
> 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. :-)

Your Honor, I plead guilty to charges as read.  ;-)

							Thanx, Paul
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