RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning

[Andrew Haley]

On Thu, 17 Oct 2024 14:28:30 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>     * 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.

This last sentence has interesting consequences for user-defined schedulers. Would it make sense to throw an exception if a carrier thread is holding a monitor while mounting a virtual thread? Doing that would also have the advantage of making some kinds of deadlock impossible.

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.

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 5341:

> 5339: 
> 5340: void MacroAssembler::inc_held_monitor_count() {
> 5341:   Address dst = Address(rthread, JavaThread::held_monitor_count_offset());

Suggestion:

// Clobbers: rscratch1 and rscratch2
void MacroAssembler::inc_held_monitor_count() {
  Address dst = Address(rthread, JavaThread::held_monitor_count_offset());

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 5357:

> 5355: 
> 5356: void MacroAssembler::dec_held_monitor_count() {
> 5357:   Address dst = Address(rthread, JavaThread::held_monitor_count_offset());

Suggestion:

// Clobbers: rscratch1 and rscratch2
void MacroAssembler::dec_held_monitor_count() {
  Address dst = Address(rthread, JavaThread::held_monitor_count_offset());

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PR Comment: https://git.openjdk.org/jdk/pull/21565#issuecomment-2429587519
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1810966647
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1810987929
PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1810989022