Date: Thu, 16 Feb 2012 07:13:19 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Mathieu Desnoyers <compudj@krystal.dyndns.org>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org,
        niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org,
        Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com,
        darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org
Subject: Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU
 implementation
Message-ID: <20120216151319.GH2976@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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 <20120216063805.GF2976@linux.vnet.ibm.com>
 <20120216110030.GA1425@Krystal>
 <1329393115.2293.204.camel@twins>
 <20120216121807.GA3426@Krystal>
 <1329396281.2293.213.camel@twins>
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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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