On Fri, 25 Oct 2024 21:33:24 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: > > - Restore use of atPointA in test StopThreadTest.java > - remove interruptible check from conditional in Object::wait Some more comments and questions on the latest commit, mostly minor. src/hotspot/share/interpreter/oopMapCache.cpp line 268: > 266: } > 267: > 268: int num_oops() { return _num_oops; } I can't find what uses this from OopMapCacheEntry. src/hotspot/share/runtime/objectMonitor.cpp line 1150: > 1148: if (LockingMode != LM_LIGHTWEIGHT && > current->is_lock_owned((address)cur)) { > 1149: assert(_recursions == 0, "invariant"); > 1150: set_owner_from_BasicLock(cur, current); // Convert from > BasicLock* to Thread*. This is nice you don't have to do this anymore. src/hotspot/share/runtime/objectMonitor.hpp line 43: > 41: // ParkEvent instead. Beware, however, that the JVMTI code > 42: // knows about ObjectWaiters, so we'll have to reconcile that code. > 43: // See next_waiter(), first_waiter(), etc. Also a nice cleanup. Did you reconcile the JVMTI code? src/hotspot/share/runtime/objectMonitor.hpp line 71: > 69: bool is_wait() { return _is_wait; } > 70: bool notified() { return _notified; } > 71: bool at_reenter() { return _at_reenter; } should these be const member functions? ------------- PR Review: https://git.openjdk.org/jdk/pull/21565#pullrequestreview-2396572570 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1817407075 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1817415918 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1817419797 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1817420178
