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

[Patricio Chilano Mateo]

On Mon, 28 Oct 2024 00:55:34 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Hmmmm ...  I guess we either slow down the monitor code by having the thread search for and remove itself, or we allow for this and handle it correctly ... okay.
>
> 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()  |                                              |

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819829472