On Thu, 24 Oct 2024 03:38:21 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: > > - Fix comment in objectMonitor.hpp and javaThread.hpp > - Skip printing tid when not available src/hotspot/share/prims/jvm.cpp line 4012: > 4010: } > 4011: ThreadBlockInVM tbivm(THREAD); > 4012: parkEvent->park(); What code does the unpark to wake this thread up? I can't quite see how this unparker thread operates as its logic seems dispersed. src/hotspot/share/runtime/javaThread.hpp line 166: > 164: // current _vthread object, except during creation of the primordial > and JNI > 165: // attached thread cases where this field can have a temporary value. > Also, > 166: // calls to VirtualThread.switchToCarrierThread will temporary change > _vthread s/temporary change/temporarily change/ src/java.base/share/classes/java/lang/Object.java line 383: > 381: try { > 382: wait0(timeoutMillis); > 383: } catch (InterruptedException e) { I had expected to see a call to a new `wait0` method that returned a value indicating whether the wait was completed or else we had to park. Instead we had to put special logic in the native-call-wrapper code in the VM to detect returning from wait0 and changing the return address. I'm still unclear where that modified return address actually takes us. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1814306675 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1814260043 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1814294622
