On Tue, 22 Oct 2024 15:37:23 GMT, Andrew Haley <a...@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... > > src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp line 60: > >> 58: >> 59: assert(LockingMode != LM_LIGHTWEIGHT, "lightweight locking should use >> fast_lock_lightweight"); >> 60: assert_different_registers(oop, box, tmp, disp_hdr, rscratch2); > > Historically, silently using `rscratch1` and `rscratch2` in these macros has > sometimes turned out to be a mistake. > Please consider making `rscratch2` an additional argument to `fast_lock`, so > that it's explicit in the caller. It won't make any difference to the > generated code, but it might help readbility. Note also that `inc_held_monitor_count` clobbers `rscratch2`. That might be worth a comment at the call site. I guess `inc_held_monitor_count` is so hot that we can't push and pop scratch registers, in which case it'd clobber nothing. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1810985771
