On Tue, 29 Oct 2024 00:04:09 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 one > additional commit since the last revision: > > Fix comment in VThreadWaitReenter src/hotspot/share/prims/jvmtiEnvBase.cpp line 1082: > 1080: } else { > 1081: assert(vthread != nullptr, "no vthread oop"); > 1082: oop oopCont = java_lang_VirtualThread::continuation(vthread); Nit: The name `oopCont` does not match the HotSpot naming convention. What about `cont_oop` or even better just `cont` as at the line 2550? src/hotspot/share/prims/jvmtiExport.cpp line 1682: > 1680: > 1681: // On preemption JVMTI state rebinding has already happened so get it > always directly from the oop. > 1682: JvmtiThreadState *state = > java_lang_Thread::jvmti_thread_state(JNIHandles::resolve(vthread)); I'm not sure this change is right. The `get_jvmti_thread_state()` has a role to lazily create a `JvmtiThreadState` if it was not created before. With this change the `JvmtiThreadState` creation can be missed if the `unmount` event is the first event encountered for this particular virtual thread. You probably remember that lazy creation of the `JvmtiThreadState`'s is an important optimization to avoid big performance overhead when a JVMTI agent is present. src/hotspot/share/prims/jvmtiExport.cpp line 2879: > 2877: > JvmtiVTMSTransitionDisabler::start_VTMS_transition((jthread)vthread.raw_value(), > /* is_mount */ true); > 2878: current->rebind_to_jvmti_thread_state_of(current->threadObj()); > 2879: } This function looks a little bit unusual. I understand it is called I need to think about the consequences but do not see anything bad so far. I'll look at the `ObjectMonitor` and `continuation` side updates to get more details on this. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1820012783 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1820052049 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1820062505
