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

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Date:	Thu, 16 Feb 2012 07:13:19 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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, Feb 16, 2012 at 01:44:41PM +0100, Peter Zijlstra wrote:
> 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 ;-)

It does look good to me, but given that I don't drink coffee, you should
take that with a large grain of salt.

							Thanx, Paul

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