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    SubjectRe: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
    On Thu, 12 Jul 2018, Andrea Parri wrote:

    > > It seems reasonable to ask people to learn that locks have stronger
    > > ordering guarantees than RMW atomics do. Maybe not the greatest
    > > situation in the world, but one I think we could live with.
    >
    > Yeah, this was one of my main objections.

    Does this mean you don't think you could live with it?

    > > > Hence my proposal to strenghten rmw-acquire, because that is the basic
    > > > primitive used to implement lock.
    > >
    > > That was essentially what the v2 patch did. (And my reasoning was
    > > basically the same as what you have just outlined. There was one
    > > additional element: smp_store_release() is already strong enough for
    > > TSO; the acquire is what needs to be stronger in the memory model.)
    >
    > Mmh? see my comments to v2 (and your reply, in part., the part "At
    > least, it's not a valid general-purpose implementation".).
    >
    >
    > > > Another, and I like this proposal least, is to introduce a new barrier
    > > > to make this all work.
    > >
    > > This apparently boils down to two questions:
    > >
    > > Should spin_lock/spin_unlock be RCsc?
    > >
    > > Should rmw-acquire be strong enough so that smp_store_release +
    > > rmw-acquire is RCtso?
    > >
    > > If both answers are No, we end up with the v3 patch. If the first
    > > answer is No and the second is Yes, we end up with the v2 patch. The
    > > problem is that different people seem to want differing answers.
    >
    > Again, maybe you're confonding v2 with v1?

    Oops, yes, I was. v1 was the version that made RMW updates be RCtso.
    v2 and v3 affected only locking, the difference being that v2 used
    unlock-rf-lock-po and v3 used po-unlock-rf-lock-po.

    Alan

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