[concurrency-interest] Ordering Question

Vitaly Davidovich vitalyd at gmail.com
Tue Jul 19 23:15:18 EDT 2016


On Tuesday, July 19, 2016, Gil Tene <gil at azul.com> wrote:

> Not an authoritative answer Mike, but:
>
> 1. Since (to my understanding) the effective StoreStore-fence-like
> behavior of lazySet/putOrdered occurs *before* the store to X below, and
> not after it, it has no effect (different from a regular store) on ordering
> in the sequence.

Yes, the storestore is before the store to X; if it was after, it would
defeat its purpose.  But as you say below, the placement of the storestore
doesn't matter in terms of preventing the volatile load from moving above
it - you need a storeLoad in between 1) and 2).

>
> 2. The a volatile read does not prevent previous stores from being
> reordered past it.
>
> As a result of these two observations, T1 can validly be reordered to:
>
> T1:
> 1) B = Y (volatile read)
> 2) X = 1 (lazySet/putOrdered)
>
> Which would then make the result you ask about quite possible:
>
> T1              T2
>
>                 Y = 1
> B = Y
>                 A = X
> X = 1
>
>
> [Note that even if the StoreStore-like-fence behavior was after the store
> (which I believe isn't the case), the reordering above would be valid,
> since it would still not prevent crossing the volatile read]


> — Gil.
>
> > On Jul 19, 2016, at 7:11 PM, Michael Barker <mikeb01 at gmail.com
> <javascript:;>> wrote:
> >
> > Hi,
> >
> > I have a question around ordering of events.
> >
> > Given, threads (T1, T2), and variables (A, B, X, Y) where X and Y are
> shared on the heap and visible to T1 and T2.
> >
> > Initially:
> >
> > X = 0
> > Y = 0;
> >
> > T1:
> > 1) X = 1 (lazySet/putOrdered)
> > 2) B = Y (volatile read)
> >
> > T2
> > 3) Y = 1 (compare and set)
> > 4) A = X (volatile read)
> >
> > Is it possible to get a final state of A = 1 and B = 0?
> >
> > My current suspicion is that it is, due to 1) and 2) being reordered.
> If so, can the final state of A=1, B=0 be prevented by strengthening 1) to
> be a volatile store?
> >
> > Regards,
> > Michael Barker.
> > _______________________________________________
> > Concurrency-interest mailing list
> > Concurrency-interest at cs.oswego.edu <javascript:;>
> > http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>
>

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