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/cpu/x86/c1_Runtime1_x86.cpp line 643: > 641: uint Runtime1::runtime_blob_current_thread_offset(frame f) { > 642: #ifdef _LP64 > 643: return r15_off / 2; r15_off is a byte offset, so this returns a 16-bit short offset? I think we need a comment here to explain the / 2 and what this returns. src/hotspot/cpu/x86/frame_x86.cpp line 431: > 429: if (cb == Runtime1::blob_for(C1StubId::monitorenter_id) || > 430: cb == Runtime1::blob_for(C1StubId::monitorenter_nofpu_id)) { > 431: thread_addr = (JavaThread**)(f.sp() + > Runtime1::runtime_blob_current_thread_offset(f)); So this expects an offset in intptr_t units from runtime_blob_current_thread_offset(), but I thought it took a byte offset and then divided by 2. I'm confused. src/hotspot/share/c1/c1_Runtime1.hpp line 138: > 136: static void initialize_pd(); > 137: > 138: static uint runtime_blob_current_thread_offset(frame f); I think this returns an offset in wordSize units, but it's not documented. In some places we always return an offset in bytes and let the caller convert. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819982432 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819983752 PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1819981522
