linux-kernel - Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU implementation

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Date:	Thu, 16 Feb 2012 15:58:22 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	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
Subject: Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU
 implementation

On Thu, 2012-02-16 at 09:52 -0500, Mathieu Desnoyers wrote:
> Ah, so this is what I missed: the context switch has its lock/unlock
> pair, the following migration is performed under its own lock/unlock
> pair, and the following context switch also has its lock/unlock pair. So
> yes, this should be sufficient to act as a full memory barrier. 

In fact it typically looks like:

 LOCK(rq0->lock)
  switch-from-A
 UNLOCK(rq0->lock);

	LOCK(rq0->lock);
	LOCK(rq1->lock);
	  migrate-A
	UNLOCK(rq1->lock);
	UNLOCK(rq0->lock);

		LOCK(rq1->lock);
		  switch-to-A
		UNLOCK(rq1->lock);

the migrate taking both locks involved guarantees that the switch-to
always comes _after_ the switch_from. The migrate might be performed on
a completely unrelated cpu although typically it would be either the old
(push) or the new (pull).
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