RFR: Invoke implDeregister() at wakeup()

[Alan Bateman]

On Mon, 4 Apr 2022 12:58:29 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> A virtual thread may park itself and register an event of EPoll when it invoke an operation of NioSocket. If the Inputstream or NioSocket closed, the call stack is like:
>> 
>> Poller::wakeup
>> Poller::stopPoll
>> NioSocketImpl.close()
>> ···
>> 
>> 
>> I have a question that do we need invoke implDeregister() before unpark virtual thread?
>
> Just to ACK that I've seen the PR. I am curious if you are seeing an issue or not. It shouldn't be necessary to explicitly deregister and I need a bit of time to page in some of the details to see if there are any side effects (async close mostly).

> @AlanBateman Thanks for your review. If we do not explicit deregister will result epoll monitor a closed fd. Because a closed fd can be reused, I have not construct a case that the number of fd monitor by epoll overflow.

Would it be possible to provide a summary of the scenario that you are concerned about? It's one shot so will be disarmed when polled. If the thread is interrupted or unparked for some other reason then it will deregister the file descriptor. If the socket is closed then it will be removed the polling mechanism (the dup2 trick isn't not used with virtual threads, in case that is what you mean).

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PR: https://git.openjdk.java.net/loom/pull/128