linux-kernel - Re: [PATCH RFC tip/core/rcu 3/4] rcu: Make RCU_FAST_NO

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Date:	Thu, 26 Apr 2012 17:09:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 3/4] rcu: Make RCU_FAST_NO_HZ account
 for pauses out of idle

On Thu, 2012-04-26 at 07:49 -0700, Paul E. McKenney wrote:
> >   per_cpu(rcu_dyntick.first_pass, cpu) = 0;

> I must confess that I had no idea that you could say the above.  I would
> have expected to have to say the following:
> 
>     per_cpu(rcu_dyntick, cpu).first_pass = 0; 

They both work and are afaik identical. The first takes the per-cpu
address of rcu_dyntick and adds the offset of first_pass, then
transforms the per-cpu address to a linear address. The second
transforms the per-cpu address of rcu_dyntick, transforms it to a linear
address and then adds the offset of first_pass, IOW:

 (&rcu_dyntick + offset) + per_cpu_offset == 
		(&rcu_dyntick + per_cpu_offset) + offset



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