On Tue, 5 Nov 2024 01:40:15 GMT, Patricio Chilano Mateo <pchilanom...@openjdk.org> wrote:
>> This is the implementation of JEP 491: Synchronize Virtual Threads without >> Pinning. See [JEP 491](https://bugs.openjdk.org/browse/JDK-8337395) for >> further details. >> >> In order to make the code review easier the changes have been split into the >> following initial 4 commits: >> >> - Changes to allow unmounting a virtual thread that is currently holding >> monitors. >> - Changes to allow unmounting a virtual thread blocked on synchronized >> trying to acquire the monitor. >> - Changes to allow unmounting a virtual thread blocked in `Object.wait()` >> and its timed-wait variants. >> - Changes to tests, JFR pinned event, and other changes in the JDK libraries. >> >> The changes fix pinning issues for all 4 ports that currently implement >> continuations: x64, aarch64, riscv and ppc. Note: ppc changes were added >> recently and stand in its own commit after the initial ones. >> >> The changes fix pinning issues when using `LM_LIGHTWEIGHT`, i.e. the default >> locking mode, (and `LM_MONITOR` which comes for free), but not when using >> `LM_LEGACY` mode. Note that the `LockingMode` flag has already been >> deprecated ([JDK-8334299](https://bugs.openjdk.org/browse/JDK-8334299)), >> with the intention to remove `LM_LEGACY` code in future releases. >> >> >> ## Summary of changes >> >> ### Unmount virtual thread while holding monitors >> >> As stated in the JEP, currently when a virtual thread enters a synchronized >> method or block, the JVM records the virtual thread's carrier platform >> thread as holding the monitor, not the virtual thread itself. This prevents >> the virtual thread from being unmounted from its carrier, as ownership >> information would otherwise go wrong. In order to fix this limitation we >> will do two things: >> >> - We copy the oops stored in the LockStack of the carrier to the stackChunk >> when freezing (and clear the LockStack). We copy the oops back to the >> LockStack of the next carrier when thawing for the first time (and clear >> them from the stackChunk). Note that we currently assume carriers don't hold >> monitors while mounting virtual threads. >> >> - For inflated monitors we now record the `java.lang.Thread.tid` of the >> owner in the ObjectMonitor's `_owner` field instead of a JavaThread*. This >> allows us to tie the owner of the monitor to a `java.lang.Thread` instance, >> rather than to a JavaThread which is only created per platform thread. The >> tid is already a 64 bit field so we can ignore issues of the counter >> wrapping around. >> >> #### General notes about this part: >> >> - Since virtual th... > > Patricio Chilano Mateo has updated the pull request incrementally with five > additional commits since the last revision: > > - Add oopDesc::has_klass_gap() check > - Rename waitTimeout/set_waitTimeout accessors > - Revert suggestion to ThawBase::new_stack_frame > - Improve JFR pinned reason in event > - Use freeze_result consistently src/hotspot/share/runtime/objectMonitor.cpp line 1643: > 1641: // actual callee (see nmethod::preserve_callee_argument_oops()). > 1642: ThreadOnMonitorWaitedEvent tmwe(current); > 1643: JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, > timed_out); We post a JVMTI `MonitorWaited` event here for a virtual thread. A couple of questions on this: - Q1: Is this posted after the `VirtualThreadMount` extension event posted? Unfortunately, it is not easy to make this conclusion. - Q2: The `JvmtiExport::post_monitor_waited()` is called at the line 1801. Does it post the `MonitorWaited` event for this virtual thread as well? If not, then it is not clear how posting for virtual thread is avoided. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1829199889
