netdev - Re: [PATCH (resubmit)][BRIDGE] Properly dereference the br_should_route

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Message-ID: <20071129234900.GB23769@gondor.apana.org.au>
Date:	Fri, 30 Nov 2007 10:49:00 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Linux Netdev List <netdev@...r.kernel.org>,
	bridge@...ts.osdl.org, devel@...nvz.org
Subject: Re: [PATCH (resubmit)][BRIDGE] Properly dereference the br_should_route_hook

On Thu, Nov 29, 2007 at 06:36:50AM -0800, Paul E. McKenney wrote:
>
> That certainly is an interesting tradeoff...  Save a memory barrier
> when assigning NULL, but pay an extra test and branch in all cases.
> Though it does make for a simpler rule -- just use rcu_assign_pointer()
> in all cases.  Of course, if almost all rcu_assign_pointer() executions
> assign non-NULL pointers, the optimal strategy would be to leave the
> implementation of rcu_assign_pointer() alone, and simply enforce use
> of rcu_assign_pointer(), even if the pointer being assigned is NULL.

I was thinking of something much simpler.  If the second argument is
constant and NULL, then skip the barrier.  No run-time slow-down at
all.
 
> Although rcu_dereference() does a memory barrier only on Alpha, that of
> rcu_assign_pointer() is needed on any machine that does not preserve store
> order (Itanium, POWER, ARM, some MIPS boxes according to rumor, ...).

Good point!

Thanks,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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