8268265: MutableSpaceUsedHelper::take_sample() hits assert(left >= right) failed: avoid overflow

Reviewed-by: tschatzl, iwalulya

src/hotspot/share/gc/parallel/spaceCounters.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2002, 2021, 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
@@ -26,6 +26,10 @@
 #include "gc/parallel/spaceCounters.hpp"
 #include "memory/allocation.inline.hpp"
 #include "memory/resourceArea.hpp"
+#include "runtime/atomic.hpp"
+#include "runtime/mutex.hpp"
+#include "runtime/mutexLocker.hpp"
+#include "utilities/debug.hpp"
 #include "utilities/macros.hpp"
 
 SpaceCounters::SpaceCounters(const char* name, int ordinal, size_t max_size,
@@ -68,3 +72,24 @@ SpaceCounters::SpaceCounters(const char* name, int ordinal, size_t max_size,
 SpaceCounters::~SpaceCounters() {
   FREE_C_HEAP_ARRAY(char, _name_space);
 }
+
+static volatile size_t last_used_in_bytes = 0;
+
+void SpaceCounters::update_used() {
+  size_t new_used = _object_space->used_in_bytes();
+  Atomic::store(&last_used_in_bytes, new_used);
+  _used->set_value(new_used);
+}
+
+jlong MutableSpaceUsedHelper::take_sample() {
+  // Sampling may occur during GC, possibly while GC is updating the space.
+  // The space can be in an inconsistent state during such an update.  We
+  // don't want to block sampling for the duration of a GC.  Instead, skip
+  // sampling in that case, using the last recorded value.
+  assert(!Heap_lock->owned_by_self(), "precondition");
+  if (Heap_lock->try_lock()) {
+    Atomic::store(&last_used_in_bytes, _m->used_in_bytes());
+    Heap_lock->unlock();
+  }
+  return Atomic::load(&last_used_in_bytes);
+}

src/hotspot/share/gc/parallel/spaceCounters.hpp

@@ -58,9 +58,7 @@ class SpaceCounters: public CHeapObj<mtGC> {
     _capacity->set_value(_object_space->capacity_in_bytes());
   }
 
-  inline void update_used() {
-    _used->set_value(_object_space->used_in_bytes());
-  }
+  void update_used();
 
   inline void update_all() {
     update_used();
@@ -77,9 +75,7 @@ class MutableSpaceUsedHelper: public PerfLongSampleHelper {
   public:
     MutableSpaceUsedHelper(MutableSpace* m) : _m(m) { }
 
-    inline jlong take_sample() {
-      return _m->used_in_bytes();
-    }
+    jlong take_sample() override;
 };
 
 #endif // SHARE_GC_PARALLEL_SPACECOUNTERS_HPP

src/hotspot/share/gc/serial/cSpaceCounters.cpp

@@ -72,8 +72,12 @@ void CSpaceCounters::update_capacity() {
   _capacity->set_value(_space->capacity());
 }
 
+static volatile size_t last_used_in_bytes = 0;
+
 void CSpaceCounters::update_used() {
-  _used->set_value(_space->used());
+  size_t new_used = _space->used();
+  Atomic::store(&last_used_in_bytes, new_used);
+  _used->set_value(new_used);
 }
 
 void CSpaceCounters::update_all() {
@@ -82,5 +86,14 @@ void CSpaceCounters::update_all() {
 }
 
 jlong ContiguousSpaceUsedHelper::take_sample(){
-  return _space->used();
+  // Sampling may occur during GC, possibly while GC is updating the space.
+  // The space can be in an inconsistent state during such an update.  We
+  // don't want to block sampling for the duration of a GC.  Instead, skip
+  // sampling in that case, using the last recorded value.
+  assert(!Heap_lock->owned_by_self(), "precondition");
+  if (Heap_lock->try_lock()) {
+    Atomic::store(&last_used_in_bytes, _space->used());
+    Heap_lock->unlock();
+  }
+  return Atomic::load(&last_used_in_bytes);
 }

src/hotspot/share/gc/serial/cSpaceCounters.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2002, 2021, 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
@@ -54,9 +54,9 @@ class CSpaceCounters: public CHeapObj<mtGC> {
 
   ~CSpaceCounters();
 
-  virtual void update_capacity();
-  virtual void update_used();
-  virtual void update_all();
+  void update_capacity();
+  void update_used();
+  void update_all();
 
   const char* name_space() const        { return _name_space; }
 };
@@ -68,7 +68,7 @@ class ContiguousSpaceUsedHelper : public PerfLongSampleHelper {
   public:
     ContiguousSpaceUsedHelper(ContiguousSpace* space) : _space(space) { }
 
-    jlong take_sample();
+    jlong take_sample() override;
 };
 
 #endif // SHARE_GC_SERIAL_CSPACECOUNTERS_HPP