--- a/hotspot/src/share/vm/memory/gcLocker.cpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/memory/gcLocker.cpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -31,38 +31,93 @@
 volatile jint GC_locker::_lock_count     = 0;
 volatile bool GC_locker::_needs_gc       = false;
 volatile bool GC_locker::_doing_gc       = false;
+jlong GC_locker::_wait_begin = 0;
+
+#ifdef ASSERT
+volatile jint GC_locker::_debug_jni_lock_count = 0;
+#endif
+
+
+#ifdef ASSERT
+void GC_locker::verify_critical_count() {
+  if (SafepointSynchronize::is_at_safepoint()) {
+    assert(!needs_gc() || _debug_jni_lock_count == _jni_lock_count, "must agree");
+    int count = 0;
+    // Count the number of threads with critical operations in progress
+    for (JavaThread* thr = Threads::first(); thr; thr = thr->next()) {
+      if (thr->in_critical()) {
+        count++;
+      }
+    }
+    if (_jni_lock_count != count) {
+      tty->print_cr("critical counts don't match: %d != %d", _jni_lock_count, count);
+      for (JavaThread* thr = Threads::first(); thr; thr = thr->next()) {
+        if (thr->in_critical()) {
+          tty->print_cr(INTPTR_FORMAT " in_critical %d", thr, thr->in_critical());
+        }
+      }
+    }
+    assert(_jni_lock_count == count, "must be equal");
+  }
+}
+#endif
+
+bool GC_locker::check_active_before_gc() {
+  assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
+  if (is_active() && !_needs_gc) {
+    verify_critical_count();
+    _needs_gc = true;
+    if (PrintJNIGCStalls && PrintGCDetails) {
+      ResourceMark rm; // JavaThread::name() allocates to convert to UTF8
+      _wait_begin = tty->time_stamp().milliseconds();
+      gclog_or_tty->print_cr(INT64_FORMAT ": Setting _needs_gc. Thread \"%s\" %d locked.",
+                             _wait_begin, Thread::current()->name(), _jni_lock_count);
+    }
+
+  }
+  return is_active();
+}
 
 void GC_locker::stall_until_clear() {
   assert(!JavaThread::current()->in_critical(), "Would deadlock");
-  if (PrintJNIGCStalls && PrintGCDetails) {
-    ResourceMark rm; // JavaThread::name() allocates to convert to UTF8
-    gclog_or_tty->print_cr(
-      "Allocation failed. Thread \"%s\" is stalled by JNI critical section.",
-      JavaThread::current()->name());
+  MutexLocker   ml(JNICritical_lock);
+
+  if (needs_gc()) {
+    if (PrintJNIGCStalls && PrintGCDetails) {
+      ResourceMark rm; // JavaThread::name() allocates to convert to UTF8
+      gclog_or_tty->print_cr(INT64_FORMAT ": Allocation failed. Thread \"%s\" is stalled by JNI critical section, %d locked.",
+                             tty->time_stamp().milliseconds() - _wait_begin, Thread::current()->name(), _jni_lock_count);
+    }
   }
-  MutexLocker   ml(JNICritical_lock);
+
   // Wait for _needs_gc  to be cleared
-  while (GC_locker::needs_gc()) {
+  while (needs_gc()) {
     JNICritical_lock->wait();
   }
 }
 
-void GC_locker::jni_lock_slow() {
+void GC_locker::jni_lock(JavaThread* thread) {
+  assert(!thread->in_critical(), "shouldn't currently be in a critical region");
   MutexLocker mu(JNICritical_lock);
   // Block entering threads if we know at least one thread is in a
   // JNI critical region and we need a GC.
   // We check that at least one thread is in a critical region before
   // blocking because blocked threads are woken up by a thread exiting
   // a JNI critical region.
-  while ((is_jni_active() && needs_gc()) || _doing_gc) {
+  while ((needs_gc() && is_jni_active()) || _doing_gc) {
     JNICritical_lock->wait();
   }
-  jni_lock();
+  thread->enter_critical();
+  _jni_lock_count++;
+  increment_debug_jni_lock_count();
 }
 
-void GC_locker::jni_unlock_slow() {
+void GC_locker::jni_unlock(JavaThread* thread) {
+  assert(thread->in_last_critical(), "should be exiting critical region");
   MutexLocker mu(JNICritical_lock);
-  jni_unlock();
+  _jni_lock_count--;
+  decrement_debug_jni_lock_count();
+  thread->exit_critical();
   if (needs_gc() && !is_jni_active()) {
     // We're the last thread out. Cause a GC to occur.
     // GC will also check is_active, so this check is not
@@ -74,11 +129,17 @@
       {
         // Must give up the lock while at a safepoint
         MutexUnlocker munlock(JNICritical_lock);
+        if (PrintJNIGCStalls && PrintGCDetails) {
+          ResourceMark rm; // JavaThread::name() allocates to convert to UTF8
+          gclog_or_tty->print_cr(INT64_FORMAT ": Thread \"%s\" is performing GC after exiting critical section, %d locked",
+                                 tty->time_stamp().milliseconds() - _wait_begin, Thread::current()->name(), _jni_lock_count);
+        }
         Universe::heap()->collect(GCCause::_gc_locker);
       }
       _doing_gc = false;
     }
-    clear_needs_gc();
+
+    _needs_gc = false;
     JNICritical_lock->notify_all();
   }
 }

--- a/hotspot/src/share/vm/memory/gcLocker.hpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/memory/gcLocker.hpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -51,53 +51,70 @@
 
 class GC_locker: public AllStatic {
  private:
-  static volatile jint _jni_lock_count;  // number of jni active instances
+  // The _jni_lock_count keeps track of the number of threads that are
+  // currently in a critical region.  It's only kept up to date when
+  // _needs_gc is true.  The current value is computed during
+  // safepointing and decremented during the slow path of GC_locker
+  // unlocking.
+  static volatile jint _jni_lock_count;  // number of jni active instances.
+
   static volatile jint _lock_count;      // number of other active instances
   static volatile bool _needs_gc;        // heap is filling, we need a GC
                                          // note: bool is typedef'd as jint
   static volatile bool _doing_gc;        // unlock_critical() is doing a GC
 
+  static jlong         _wait_begin;      // Timestamp for the setting of _needs_gc.
+                                         // Used only by printing code.
+
+#ifdef ASSERT
+  // This lock count is updated for all operations and is used to
+  // validate the jni_lock_count that is computed during safepoints.
+  static volatile jint _debug_jni_lock_count;
+#endif
+
   // Accessors
   static bool is_jni_active() {
+    assert(_needs_gc, "only valid when _needs_gc is set");
     return _jni_lock_count > 0;
   }
 
-  static void set_needs_gc() {
-    assert(SafepointSynchronize::is_at_safepoint(),
-      "needs_gc is only set at a safepoint");
-    _needs_gc = true;
-  }
-
-  static void clear_needs_gc() {
-    assert_lock_strong(JNICritical_lock);
-    _needs_gc = false;
-  }
+  // At a safepoint, visit all threads and count the number of active
+  // critical sections.  This is used to ensure that all active
+  // critical sections are exited before a new one is started.
+  static void verify_critical_count() NOT_DEBUG_RETURN;
 
-  static void jni_lock() {
-    Atomic::inc(&_jni_lock_count);
-    CHECK_UNHANDLED_OOPS_ONLY(
-      if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count++; })
-    assert(Universe::heap() == NULL || !Universe::heap()->is_gc_active(),
-           "locking failed");
-  }
-
-  static void jni_unlock() {
-    Atomic::dec(&_jni_lock_count);
-    CHECK_UNHANDLED_OOPS_ONLY(
-      if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count--; })
-  }
-
-  static void jni_lock_slow();
-  static void jni_unlock_slow();
+  static void jni_lock(JavaThread* thread);
+  static void jni_unlock(JavaThread* thread);
 
  public:
   // Accessors
   static bool is_active();
   static bool needs_gc()       { return _needs_gc;                        }
+
   // Shorthand
-  static bool is_active_and_needs_gc() { return is_active() && needs_gc();}
+  static bool is_active_and_needs_gc() { return needs_gc() && is_active(); }
 
-  // Calls set_needs_gc() if is_active() is true. Returns is_active().
+  // In debug mode track the locking state at all times
+  static void increment_debug_jni_lock_count() {
+#ifdef ASSERT
+    assert(_debug_jni_lock_count >= 0, "bad value");
+    Atomic::inc(&_debug_jni_lock_count);
+#endif
+  }
+  static void decrement_debug_jni_lock_count() {
+#ifdef ASSERT
+    assert(_debug_jni_lock_count > 0, "bad value");
+    Atomic::dec(&_debug_jni_lock_count);
+#endif
+  }
+
+  // Set the current lock count
+  static void set_jni_lock_count(int count) {
+    _jni_lock_count = count;
+    verify_critical_count();
+  }
+
+  // Sets _needs_gc if is_active() is true. Returns is_active().
   static bool check_active_before_gc();
 
   // Stalls the caller (who should not be in a jni critical section)
@@ -131,20 +148,20 @@
   // JNI critical regions are the only participants in this scheme
   // because they are, by spec, well bounded while in a critical region.
   //
-  // Each of the following two method is split into a fast path and a slow
-  // path. JNICritical_lock is only grabbed in the slow path.
+  // Each of the following two method is split into a fast path and a
+  // slow path. JNICritical_lock is only grabbed in the slow path.
   // _needs_gc is initially false and every java thread will go
-  // through the fast path (which does the same thing as the slow path
-  // when _needs_gc is false). When GC happens at a safepoint,
-  // GC_locker::is_active() is checked. Since there is no safepoint in the
-  // fast path of lock_critical() and unlock_critical(), there is no race
-  // condition between the fast path and GC. After _needs_gc is set at a
-  // safepoint, every thread will go through the slow path after the safepoint.
-  // Since after a safepoint, each of the following two methods is either
-  // entered from the method entry and falls into the slow path, or is
-  // resumed from the safepoints in the method, which only exist in the slow
-  // path. So when _needs_gc is set, the slow path is always taken, till
-  // _needs_gc is cleared.
+  // through the fast path, which simply increments or decrements the
+  // current thread's critical count.  When GC happens at a safepoint,
+  // GC_locker::is_active() is checked. Since there is no safepoint in
+  // the fast path of lock_critical() and unlock_critical(), there is
+  // no race condition between the fast path and GC. After _needs_gc
+  // is set at a safepoint, every thread will go through the slow path
+  // after the safepoint.  Since after a safepoint, each of the
+  // following two methods is either entered from the method entry and
+  // falls into the slow path, or is resumed from the safepoints in
+  // the method, which only exist in the slow path. So when _needs_gc
+  // is set, the slow path is always taken, till _needs_gc is cleared.
   static void lock_critical(JavaThread* thread);
   static void unlock_critical(JavaThread* thread);
 };

--- a/hotspot/src/share/vm/memory/gcLocker.inline.hpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/memory/gcLocker.inline.hpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -28,16 +28,11 @@
 #include "memory/gcLocker.hpp"
 
 inline bool GC_locker::is_active() {
+  assert(_needs_gc || SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
+  verify_critical_count();
   return _lock_count > 0 || _jni_lock_count > 0;
 }
 
-inline bool GC_locker::check_active_before_gc() {
-  if (is_active()) {
-    set_needs_gc();
-  }
-  return is_active();
-}
-
 inline void GC_locker::lock() {
   // cast away volatile
   Atomic::inc(&_lock_count);
@@ -56,24 +51,28 @@
 
 inline void GC_locker::lock_critical(JavaThread* thread) {
   if (!thread->in_critical()) {
-    if (!needs_gc()) {
-      jni_lock();
-    } else {
-      jni_lock_slow();
+    if (needs_gc()) {
+      // jni_lock call calls enter_critical under the lock so that the
+      // global lock count and per thread count are in agreement.
+      jni_lock(thread);
+      return;
     }
+    increment_debug_jni_lock_count();
   }
   thread->enter_critical();
 }
 
 inline void GC_locker::unlock_critical(JavaThread* thread) {
+  if (thread->in_last_critical()) {
+    if (needs_gc()) {
+      // jni_unlock call calls exit_critical under the lock so that
+      // the global lock count and per thread count are in agreement.
+      jni_unlock(thread);
+      return;
+    }
+    decrement_debug_jni_lock_count();
+  }
   thread->exit_critical();
-  if (!thread->in_critical()) {
-    if (!needs_gc()) {
-      jni_unlock();
-    } else {
-      jni_unlock_slow();
-    }
-  }
 }
 
 #endif // SHARE_VM_MEMORY_GCLOCKER_INLINE_HPP

--- a/hotspot/src/share/vm/runtime/safepoint.cpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/runtime/safepoint.cpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -95,6 +95,7 @@
 SafepointSynchronize::SynchronizeState volatile SafepointSynchronize::_state = SafepointSynchronize::_not_synchronized;
 volatile int  SafepointSynchronize::_waiting_to_block = 0;
 volatile int SafepointSynchronize::_safepoint_counter = 0;
+int SafepointSynchronize::_current_jni_active_count = 0;
 long  SafepointSynchronize::_end_of_last_safepoint = 0;
 static volatile int PageArmed = 0 ;        // safepoint polling page is RO|RW vs PROT_NONE
 static volatile int TryingToBlock = 0 ;    // proximate value -- for advisory use only
@@ -138,6 +139,9 @@
   {
   MutexLocker mu(Safepoint_lock);
 
+  // Reset the count of active JNI critical threads
+  _current_jni_active_count = 0;
+
   // Set number of threads to wait for, before we initiate the callbacks
   _waiting_to_block = nof_threads;
   TryingToBlock     = 0 ;
@@ -375,6 +379,9 @@
 
   OrderAccess::fence();
 
+  // Update the count of active JNI critical regions
+  GC_locker::set_jni_lock_count(_current_jni_active_count);
+
   if (TraceSafepoint) {
     VM_Operation *op = VMThread::vm_operation();
     tty->print_cr("Entering safepoint region: %s", (op != NULL) ? op->name() : "no vm operation");
@@ -585,6 +592,11 @@
         _waiting_to_block--;
         thread->safepoint_state()->set_has_called_back(true);
 
+        if (thread->in_critical()) {
+          // Notice that this thread is in a critical section
+          increment_jni_active_count();
+        }
+
         // Consider (_waiting_to_block < 2) to pipeline the wakeup of the VM thread
         if (_waiting_to_block == 0) {
           Safepoint_lock->notify_all();
@@ -861,8 +873,12 @@
   // running, but are actually at a safepoint. We will happily
   // agree and update the safepoint state here.
   if (SafepointSynchronize::safepoint_safe(_thread, state)) {
-      roll_forward(_at_safepoint);
-      return;
+    roll_forward(_at_safepoint);
+    if (_thread->in_critical()) {
+      // Notice that this thread is in a critical section
+      SafepointSynchronize::increment_jni_active_count();
+    }
+    return;
   }
 
   if (state == _thread_in_vm) {

--- a/hotspot/src/share/vm/runtime/safepoint.hpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/runtime/safepoint.hpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -29,6 +29,7 @@
 #include "code/nmethod.hpp"
 #include "memory/allocation.hpp"
 #include "runtime/extendedPC.hpp"
+#include "runtime/mutexLocker.hpp"
 #include "runtime/os.hpp"
 #include "utilities/ostream.hpp"
 
@@ -92,6 +93,7 @@
  private:
   static volatile SynchronizeState _state;     // Threads might read this flag directly, without acquireing the Threads_lock
   static volatile int _waiting_to_block;       // number of threads we are waiting for to block
+  static int _current_jni_active_count;        // Counts the number of active critical natives during the safepoint
 
   // This counter is used for fast versions of jni_Get<Primitive>Field.
   // An even value means there is no ongoing safepoint operations.
@@ -146,6 +148,11 @@
     return (_state != _not_synchronized);
   }
 
+  inline static void increment_jni_active_count() {
+    assert_locked_or_safepoint(Safepoint_lock);
+    _current_jni_active_count++;
+  }
+
   // Called when a thread volantary blocks
   static void   block(JavaThread *thread);
   static void   signal_thread_at_safepoint()              { _waiting_to_block--; }

--- a/hotspot/src/share/vm/runtime/thread.hpp	Thu Jan 26 19:39:08 2012 -0800
+++ b/hotspot/src/share/vm/runtime/thread.hpp	Sun Jan 29 16:46:04 2012 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -1310,6 +1310,7 @@
 
   // JNI critical regions. These can nest.
   bool in_critical()    { return _jni_active_critical > 0; }
+  bool in_last_critical()  { return _jni_active_critical == 1; }
   void enter_critical() { assert(Thread::current() == this,
                                  "this must be current thread");
                           _jni_active_critical++; }