RFR: 8305403: Shenandoah evacuation workers may deadlock

[William Kemper]

Shenandoah evacuation workers may deadlock if a safepoint begins during concurrent evacuation _and_ a GC worker thread experiences an out of memory error during the evacuation.

This situation occurs because some of the workers have set the cancelled state to `NOT_CANCELLED` and then yielded to the suspendible thread set because a safepoint is starting. Other workers who then experience an OOM during evacuation attempt to transition the heap from `CANCELLABLE` to `CANCELLED` in a CAS loop that will never succeed (because the cancelled state is `NOT_CANCELLED`). These workers are unable to join the suspendible thread set, so the threads which have yielded are unable to resume and reset the heap to `CANCELLABLE`. The VM thread cannot enter the safepoint and eventually all of the mutator threads block when they are unable to allocate.

The changes here remove the `NOT_CANCELLED` state and remove the CAS loop used to transition from `CANCELLABLE` to `CANCELLED`. Additionally, worker threads that need to suspend no longer put the heap in the `NOT_CANCELLED` state before suspending.

In order to test this, the behavior of the diagnostic flag was modified to provoke this scenario:

diff --git a/src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp b/src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp
index 4158f4bee22..e261dd3a81b 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp
@@ -292,8 +292,7 @@ inline oop ShenandoahHeap::evacuate_object(oop p, Thread* thread) {
   HeapWord* copy = nullptr;
 
 #ifdef ASSERT
-  if (ShenandoahOOMDuringEvacALot &&
-      (os::random() & 1) == 0) { // Simulate OOM every ~2nd slow-path call
+  if (ShenandoahOOMDuringEvacALot && thread->is_Worker_thread() && SuspendibleThreadSet::should_yield() && uint(os::random()) % 1000 < 1) {
         copy = nullptr;
   } else {
 #endif


This applies the failure only to worker threads and only when they are expected to suspend and only with a 1/1000 chance of failing for each evacuation. Without these fixes, the modification to `ShenandoahOOMDuringEvacALot` causes a deadlock in under two minutes of execution on [the extremem benchmark](https://github.com/corretto/heapothesys/tree/master/Extremem) with these additional diagnostic flags:

-XX:+SafepointALot -XX:+ShenandoahOOMDuringEvacALot -XX:ConcGCThreads=12 -XX:GuaranteedSafepointInterval=50

After these changes, the benchmark no longer deadlocks.

Additional testing includes jtreg `test/hotspot/jtreg:hotspot_gc_shenandoah`, the Dacapo benchmark suite, HyperAlloc and specjbb 2015.

-------------

Commit messages:
 - Enable suspendible workers by default
 - Don't suspend if GC is already cancelled
 - Do not prevent cancellations before yielding to STS. Simplify cancellation states

Changes: https://git.openjdk.org/jdk/pull/13309/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305403
  Stats: 39 lines in 6 files changed: 1 ins; 33 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13309.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13309/head:pull/13309

PR: https://git.openjdk.org/jdk/pull/13309