RFR (XS): 8188877: Improper synchronization in offer_termination

Andrew Haley aph at redhat.com
Mon Nov 27 15:57:25 UTC 2017

On 27/11/17 15:44, Andrew Dinn wrote:
> On 27/11/17 14:53, Andrew Haley wrote:
>> On 27/11/17 12:30, Andrew Dinn wrote:
> <snip>
>>> That's what happens when the reader executes a read barrier. The
>>> interesting question is what happens when the reader does not execute a
>>> read barrier.
>> The invalidate messages still arrive at the reader, but they sit in
>> the invalidate queue and aren't acted upon immediately.  Eventually
>> they must be processed, either lazily or because the reader's
>> invalidate queue fills up.
> Hmm, that explanation assumes there will be other invalidate messages.

No, not at all.  By "lazily" I mean that while a core has nothing else
to do it might as well process its invalidate queue, and AFAIK that is
what happens.

> But at a STW pause that's not necessarily going to be the case. In the
> worst case all other threads may could be spinning on a barrier count
> while this one core/thread has a single invalidate message in its queue.

That could be, but there are other things go on.  There are other threads
active, and invalidate messages get sent to everyone.

In practice, I've never seen more than a few microseconds of delay.

Bear in mind that the interpreter changes we just made mean that
interpreted code won't necessarily see safepoint status changes for
about 100 microseconds, so the lack of an acquiring load in our
code is really not the biggest issue.

Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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