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Hi Carsten,<br>
<br>
I believe Kim is referring precisely to the release in the fence
before the compare-and-exchange in that comment you refer to, rather
than the membar Store* after the fence.<br>
I would prefer to have explicit memory ordering between the linked
producer and consumer pair, as Kim suggests. This is how the JMM
implements sequential consistency - cooperatively with
release_store_fence and load_acquire pairs on the same data. It
might very well be that specific architectures might work fine
without the acquire for the consumer as the producer implementation
itself is strong enough to guarantee sequential consistency, but I
don't see why it would be wise to bet on that if we don't have to,
and it does not cost us anything.<br>
<br>
Thanks,<br>
/Erik<br>
<br>
<div class="moz-cite-prefix">On 2016-09-19 21:36, Carsten Varming
wrote:<br>
</div>
<blockquote
cite="mid:CAP_pwnU+NWhtZ1tmtMoZ7sydA28QcRfY0e7piVJsFFAgq_QBXw@mail.gmail.com"
type="cite">
<div dir="ltr">Dear Kim,
<div><br>
</div>
<div>Reading a volatile field updated by cmpxchg does not need
an acquire operation.</div>
<div><br>
</div>
<div>BTW. The specification in atomic.hpp says cmpxchg provides:
"<fence> compare-and-exchange <membar
StoreLoad|StoreStore>". The release is part of the fence.
Are you suggesting that there is a release after the exchange?</div>
<div><br>
</div>
<div>Carsten</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Sep 19, 2016 at 3:18 PM, Kim
Barrett <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:kim.barrett@oracle.com" target="_blank">kim.barrett@oracle.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="HOEnZb">
<div class="h5">> On Sep 19, 2016, at 2:26 PM, Carsten
Varming <<a moz-do-not-send="true"
href="mailto:varming@gmail.com">varming@gmail.com</a>>
wrote:<br>
><br>
> Dear Erik,<br>
><br>
> According to orderAccess.hpp an acquire is supposed
to be paired with a release. It doesn't look like there
is any synchronization on the data written to the time
stamp array, so what exactly is going on?<br>
><br>
> Carsten<br>
<br>
</div>
</div>
_time_stamps gets written with a cmpxchg_ptr (line 63 of
gcTaskThread.cpp), which includes release semantics for the
write; see atomic.hpp.<br>
<br>
</blockquote>
</div>
<br>
</div>
</blockquote>
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