Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCUimplementation

[Peter Zijlstra]

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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