fix race condition in interrupting/joining threads as they are exiting

My recent commit to ensure that OS resources are released immediately
upon thread exit introduced a race condition where interrupting or
joining a thread as it exited could lead to attempts to use
already-released resources.  This commit adds locking to avoid the
race.

src/machine.cpp

@@ -45,10 +45,9 @@ void
 join(Thread* t, Thread* o)
 {
   if (t != o) {
-    if (o->state != Thread::ZombieState
+    if (acquireSystem(t, o)) {
-        and o->state != Thread::JoinedState)
-    {
       o->systemThread->join();
+      releaseSystem(t, o);
     }
     o->state = Thread::JoinedState;
   }
@@ -2407,7 +2406,15 @@ Thread::exit()
       turnOffTheLights(this);
     } else {
       threadPeer(this, javaThread) = 0;
+
+      { ACQUIRE_RAW(this, m->stateLock);
+
+        while (flags & SystemFlag) {
+          m->stateLock->wait(systemThread, 0);
+        }
+      
         enter(this, Thread::ZombieState);
+      }
 
       lock->dispose();
       lock = 0;

src/machine.h

@@ -1298,6 +1298,7 @@ class Thread {
   static const unsigned DaemonFlag = 1 << 3;
   static const unsigned StressFlag = 1 << 4;
   static const unsigned ActiveFlag = 1 << 5;
+  static const unsigned SystemFlag = 1 << 6;
 
   class Protector {
    public:
@@ -2758,13 +2759,42 @@ notifyAll(Thread* t, object o)
   }
 }
 
+inline bool
+zombified(Thread* t)
+{
+  return t->state == Thread::ZombieState
+    or t->state == Thread::JoinedState;
+}
+
+inline bool
+acquireSystem(Thread* t, Thread* target)
+{
+  ACQUIRE_RAW(t, t->m->stateLock);
+
+  if (not zombified(target)) {
+    atomicOr(&(target->flags), Thread::SystemFlag);
+    return true;
+  } else {
+    return false;
+  }
+}
+
 inline void
-interrupt(Thread*, Thread* target)
+releaseSystem(Thread* t, Thread* target)
 {
-  if (target->state != Thread::ZombieState
+  ACQUIRE_RAW(t, t->m->stateLock);
-      and target->state != Thread::JoinedState)
+
+  assert(t, not zombified(target));
+
+  atomicAnd(&(target->flags), ~Thread::SystemFlag);
+}
+
+inline void
+interrupt(Thread* t, Thread* target)
 {
+  if (acquireSystem(t, target)) {
     target->systemThread->interrupt();
+    releaseSystem(t, target);
   }
 }