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SubjectRe: [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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