RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v7]

[David Holmes]

On Fri, 25 Oct 2024 18:36:50 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/VirtualThread.java line 952:
>> 
>>> 950:         for (;;) {
>>> 951:             boolean unblocked = false;
>>> 952:             synchronized (timedWaitLock()) {
>> 
>> Where is the overall design of the timed-wait protocol and it use of synchronization described?
>
> When we unmount on a timed-wait call we schedule a wakeup task at the end of `afterYield`. There are two mechanisms that avoid the scheduled task to run and wake up the virtual thread on a future timed-wait call, since in this call the virtual thread could have been already notified before the scheduled task runs. The first one is to cancel the scheduled task once we return from the wait call (see `Object.wait(long timeoutMillis)`). Since the task could have been already started though, we also use `timedWaitSeqNo`, which the wake up task checks here to make sure it is not an old one. Since we synchronize on `timedWaitLock` to increment `timedWaitSeqNo` and change state to `TIMED_WAIT` before scheduling the wake up task in `afterYield`, here either a wrong `timedWaitSeqNo` or a state different than `TIMED_WAIT` means there is nothing to do. The only exception is checking for `SUSPENDED` state, in which case we just loop to retry.

Thanks for the explanation but that needs to be documented somewhere.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1818228510