8292194: G1 nmethod entry barrier disarm value wraps around too early

Reviewed-by: ayang, tschatzl

src/hotspot/share/gc/shared/barrierSetNMethod.cpp

@@ -117,8 +117,14 @@ class BarrierSetNMethodArmClosure : public ThreadClosure {
 void BarrierSetNMethod::arm_all_nmethods() {
   // Change to a new global GC phase. Doing this requires changing the thread-local
   // disarm value for all threads, to reflect the new GC phase.
+  // We wrap around at INT_MAX. That means that we assume nmethods won't have ABA
+  // problems in their nmethod disarm values after INT_MAX - 1 GCs. Every time a GC
+  // completes, ABA problems are removed, but if a concurrent GC is started and then
+  // aborted N times, that is when there could be ABA problems. If there are anything
+  // close to INT_MAX - 1 GCs starting without being able to finish, something is
+  // seriously wrong.
   ++_current_phase;
-  if (_current_phase == 4) {
+  if (_current_phase == INT_MAX) {
     _current_phase = 1;
   }
   BarrierSetNMethodArmClosure cl(_current_phase);