On Thu, 31 Oct 2024 21:50:50 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 two > additional commits since the last revision: > > - add comment to ThreadService::find_deadlocks_at_safepoint > - Remove assignments in preempt_kind enum Marked as reviewed by dholmes (Reviewer). src/hotspot/share/runtime/continuationFreezeThaw.cpp line 889: > 887: return f.is_native_frame() ? recurse_freeze_native_frame(f, caller) > : recurse_freeze_stub_frame(f, caller); > 888: } else { > 889: // frame can't be freezed. Most likely the call_stub or upcall_stub Suggestion: // Frame can't be frozen. Most likely the call_stub or upcall_stub src/hotspot/share/services/threadService.cpp line 467: > 465: if (waitingToLockMonitor->has_owner()) { > 466: currentThread = Threads::owning_thread_from_monitor(t_list, > waitingToLockMonitor); > 467: // If currentThread is nullptr we would like to know if the > owner Suggestion: // If currentThread is null we would like to know if the owner src/hotspot/share/services/threadService.cpp line 474: > 472: // vthread we never record this as a deadlock. Note: unless > there > 473: // is a bug in the VM, or a thread exits without releasing > monitors > 474: // acquired through JNI, nullptr should imply unmounted > vthread owner. Suggestion: // acquired through JNI, null should imply an unmounted vthread owner. ------------- PR Review: https://git.openjdk.org/jdk/pull/21565#pullrequestreview-2409348761 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1825344054 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1825344940 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1825345446
