RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning

[David Holmes]

On Mon, 28 Oct 2024 22:02:02 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> That said such a scenario is not about concurrently pushing the same thread to the list from different threads. So I'm still somewhat confused about the concurrency control here. Specifically I can't see how the cmpxchg on line 2090 could fail.
>
> Let's say ThreadA owns monitorA and ThreadB owns monitorB, here is how the cmpxchg could fail:
> 
> |                ThreadA                |                   ThreadB             |           ThreadC             |
> | --------------------------------------| --------------------------------------| ---------------------------------------------|
> |                                       |                                       |VThreadMonitorEnter:fails to acquire monitorB |
> |                                       |                                       |    VThreadMonitorEnter:adds to B's _cxq      |
> |                                       | ExitEpilog:picks ThreadC as succesor  |                                              |
> |                                       | ExitEpilog:releases monitorB          |                                              |
> |                                       |                                       |   VThreadMonitorEnter:acquires monitorB      |
> |                                       |                                       |   VThreadMonitorEnter:removes from B's _cxq  |
> |                                       |                                       |     continues execution in Java              |
> |                                       |                                       |VThreadMonitorEnter:fails to acquire monitorA |
> |                                       |                                       |    VThreadMonitorEnter:adds to A's _cxq      |
> |  ExitEpilog:picks ThreadC as succesor |                                       |                                              |
> |  ExitEpilog:releases monitorA         |                                       |                                              |
> |  ExitEpilog:calls set_onWaitingList() | ExitEpilog:calls set_onWaitingList()  |                                              |

Thanks for that detailed explanation. It is a bit disconcerting that Thread C could leave a trace on monitors it acquired and released in the distant past. But that is an effect of waking the successor after releasing the monitor (which is generally a good thing for performance). We could potentially re-check the successor (which Thread C will clear) before doing the actual unpark (and set_onWaitingList) but that would just narrow the race window not close it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1823394886