linux-kernel - Re: [PATCH] An RCU for SMP with a single CPU garbage collector

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Message-ID: <20110307213349.GA27648@tsunami.ccur.com>
Date:	Mon, 7 Mar 2011 16:33:49 -0500
From:	Joe Korty <joe.korty@...r.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] An RCU for SMP with a single CPU garbage collector

On Mon, Mar 07, 2011 at 04:16:13PM -0500, Korty, Joe wrote:
> On Mon, Mar 07, 2011 at 04:01:57PM -0500, Paul E. McKenney wrote:
>> If not, what is
>> doing the RCU read-side critical sections on the dedicated CPUs?

Oops, forgot to answer this.  RCU critical regions are
delimited by preempt_enable ... preempt_disable.  There is
a tie-in to preempt_disable(): it sets a special per-cpu
variable to zero whenever preempt_count() goes to zero.

The per-cpu variables are all periodically examined by
the the global garbage collector.  The current batch ends
when all of the per-cpu variables have gone to zero.  It
then resets each to 1 or to 0, depending on the current
state of the corresponding cpu.

Regards,
Joe
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