RFR: 8266432: ZGC: GC allocation stalls can trigger deadlocks

[Stefan Karlsson]

A deadlock can happen when a relocating thread holds the lock and tries to log information about the current thread, which can trigger a load barrier and a secondary relocation. The first relocation is holding the _ref_lock and the second relocation hangs when trying to reacquiring it. This is the stack trace:

#1 0x00007ff375c1fc90 in os::PlatformMonitor::wait
#2 0x00007ff3760cbf92 in ZForwarding::wait_page_released
#3 0x00007ff376118065 in ZRelocate::relocate_object
#4 0x00007ff374f7097b in AccessInternal::PostRuntimeDispatch<ZBarrierSet::AccessBarrier<286790ul, ZBarrierSet>,
#5 0x00007ff375176134 in oopDesc::obj_field
#6 0x00007ff37551c5cb in java_lang_Thread::name
#7 0x00007ff375f2e7ee in JavaThread::get_thread_name_string
#8 0x00007ff376126323 in ZStatPhase::log_end
#9 0x00007ff3761272e8 in ZStatCriticalPhase::register_end
#10 0x00007ff3760cc0b0 in ZForwarding::wait_page_released
#11 0x00007ff376118065 in ZRelocate::relocate_object
#12 0x00007ff3760989c5 in ZLoadBarrierOopClosure::do_oop
#13 0x00007ff375408ca8 in HandleArea::oops_do
#14 0x00007ff375f2e0e9 in JavaThread::oops_do_no_frames

This started to happen after:
8261759: ZGC: ZWorker Threads Continue Marking After System.exit() called 

The proposed patch moves the scope of the logging to outside the lock scope. The first _ref_lock check isn't really necessary. It was introduced to limit the allocation stall logging, but if a thread entered wait_page_released it really was on its way to stall.

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Commit messages:
 - 8266432: ZGC: GC allocation stalls can trigger deadlocks

Changes: https://git.openjdk.java.net/jdk/pull/3839/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3839&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266432
  Stats: 6 lines in 1 file changed: 1 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3839.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3839/head:pull/3839

PR: https://git.openjdk.java.net/jdk/pull/3839