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SubjectRe: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU implementation
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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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