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

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Date:	Thu, 16 Feb 2012 13:44:41 +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 07:18 -0500, Mathieu Desnoyers wrote:
> 
> Hrm, I think we'd need a little more than just lock/unlock ordering
> guarantees. Let's consider the following, where the stores would be
> expected to be seen as "store A before store B" by CPU 2
> 
> CPU 0             CPU 1               CPU 2
> 
>                                       load B, smp_rmb, load A in loop,
>                                       expecting that when updated A is
>                                       observed, B is always observed as
>                                       updated too.
> store A
> (lock is permeable:
> outside can leak
> inside)
> lock(rq->lock)
> 
>       -> migration ->
> 
>                   unlock(rq->lock)
>                   (lock is permeable:
>                   outside can leak inside)
>                   store B

You got the pairing the wrong way around, I suggested:

  store A

  switch-out
    UNLOCK

  	-> migration ->

			switch-in
			  LOCK

			store B

While both LOCK and UNLOCK are semi-permeable, A won't pass the UNLOCK
and B won't pass the LOCK.

Yes, A can pass switch-out LOCK, but that doesn't matter much since the
switch-in cannot happen until we've passed UNLOCK.

And yes B can pass switch-in UNLOCK, but again, I can't see that being a
problem since the LOCK will avoid it being visible before A.

> Does that make sense, or should I get my first morning coffee ? :) 

Probably.. but that's not saying I'm not wrong ;-)
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