linux-kernel - Re: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection

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Date:	Thu, 06 Sep 2012 16:36:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <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,
	sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state
 detection

On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> 
> The current quiescent-state detection algorithm is needlessly
> complex.

Heh! Be careful, we might be led into believing all this RCU is actually
really rather simple and this complexity is a bug on your end ;-)

>   It records the grace-period number corresponding to
> the quiescent state at the time of the quiescent state, which
> works, but it seems better to simply erase any record of previous
> quiescent states at the time that the CPU notices the new grace
> period.  This has the further advantage of removing another piece
> of RCU for which lockless reasoning is required. 

So why didn't you do that from the start? :-)

That is, I'm curious to know some history, why was it so and what led
you to this insight?
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