[jmm-dev] weakCompareAndSet memory semantics

David Holmes david.holmes at oracle.com
Mon Apr 25 01:54:56 UTC 2016

On 25/04/2016 8:40 AM, Martin Buchholz wrote:
> It's time for me to understand this. Perhaps by explaining it to
> others in public?
> On a LL/SC platform with spurious failures, strong cas has to be
> implemented in a loop
> boolean strong_cas(address, expect, update) {
>    for (;;) {
>      if (load_linked(address) != expect) return false;
>      if (store_conditional(address, update)) return true;
>    }
> }
> while weak cas can do the straightforward
> boolean weak_cas(address, expect, update) {
>      return load_linked(address) == expect && store_conditional(address, update);
> }
> If the store_conditional in strong_cas fails, it's probably because of
> true contention and not a spurious failure, so the extra load_linked
> and compare on retry is very likely wasted effort.  Especially

Thanks Martin. I have completely overlooked the fact that we have to 
re-load and re-compare to distinguish be a spurious and real failure.


> because, unlike C++, we don't return the value we read to the caller,
> so they will probably uselessly re-read.  Hotspot could optimize away
> the useless read, but I would be surprised if it does.
> So we should rewrite every cas loop to use weak cas!  Except, to muddy
> the waters ...
> There's two kinds of weakness - spurious failure, and relaxed memory order.
> The "weak cas" methods in Unsafe.java have no documentation, so we
> don't know what kind of weakness!
> The old public methods in j.u.c.atomic have documentation, but they
> are weak in two different ways!
> """May fail spuriously and does not provide ordering guarantees"""
> https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/AtomicLong.html#weakCompareAndSet-long-long-
> but we (probably) want a variant that "may fail spuriously" but still
> provides sequential consistency, for use in our CAS loops.

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