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From: David Miller <davem@davemloft.net>
Subject: Re: cat /proc/net/tcp takes 0.5 seconds on x86_64
Date: Thu, 28 Aug 2008 00:59:09 -0700 (PDT)
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From: Andi Kleen <andi@firstfloor.org>
Date: Thu, 28 Aug 2008 09:45:36 +0200

> > When scanning route cache hash table, we can avoid taking locks for empty 
> > buckets.
> 
> I'm not sure it's worth it in this case.  A rcu_read_lock() is a nop
> (on non preemptible kernel) to very cheap (non atomic increment/decrement in 
> cached task_struct) 

It's not one, it's at least two such increments, plus function calls,
state checks, etc. since this is rcu_read_lock_bh().