8251179: Word tearing problem with _last_sweep

Reviewed-by: coleenp, dholmes, kbarrett

src/hotspot/share/jfr/leakprofiler/checkpoint/eventEmitter.cpp

@@ -74,7 +74,7 @@ size_t EventEmitter::write_events(ObjectSampler* object_sampler, EdgeStore* edge
   assert(object_sampler != NULL, "invariant");
   assert(edge_store != NULL, "invariant");
 
-  const jlong last_sweep = emit_all ? max_jlong : object_sampler->last_sweep().value();
+  const jlong last_sweep = emit_all ? max_jlong : ObjectSampler::last_sweep();
   size_t count = 0;
 
   // First pass associates a live sample with its immediate edge

src/hotspot/share/jfr/leakprofiler/checkpoint/objectSampleCheckpoint.cpp

@@ -133,7 +133,7 @@ int ObjectSampleCheckpoint::save_mark_words(const ObjectSampler* sampler, Object
   if (sampler->last() == NULL) {
     return 0;
   }
-  SampleMarker sample_marker(marker, emit_all ? max_jlong : sampler->last_sweep().value());
+  SampleMarker sample_marker(marker, emit_all ? max_jlong : ObjectSampler::last_sweep());
   iterate_samples(sample_marker, true);
   return sample_marker.count();
 }
@@ -361,7 +361,7 @@ class BlobWriter {
 
 static void write_sample_blobs(const ObjectSampler* sampler, bool emit_all, Thread* thread) {
   // sample set is predicated on time of last sweep
-  const jlong last_sweep = emit_all ? max_jlong : sampler->last_sweep().value();
+  const jlong last_sweep = emit_all ? max_jlong : ObjectSampler::last_sweep();
   JfrCheckpointWriter writer(thread, false);
   BlobWriter cbw(sampler, writer, last_sweep);
   iterate_samples(cbw, true);

src/hotspot/share/jfr/leakprofiler/sampling/objectSampler.cpp

@@ -34,6 +34,7 @@
 #include "jfr/recorder/checkpoint/jfrCheckpointManager.hpp"
 #include "jfr/recorder/stacktrace/jfrStackTraceRepository.hpp"
 #include "jfr/support/jfrThreadLocal.hpp"
+#include "jfr/utilities/jfrTime.hpp"
 #include "jfr/utilities/jfrTryLock.hpp"
 #include "logging/log.hpp"
 #include "memory/universe.hpp"
@@ -44,7 +45,8 @@
 #include "runtime/thread.hpp"
 
 // Timestamp of when the gc last processed the set of sampled objects.
-static JfrTicks _last_sweep;
+// Atomic access to prevent word tearing on 32-bit platforms.
+static volatile int64_t _last_sweep;
 
 // Condition variable to communicate that some sampled objects have been cleared by the gc
 // and can therefore be removed from the sample priority queue.
@@ -66,7 +68,7 @@ void ObjectSampler::oop_storage_gc_notification(size_t num_dead) {
     // instance was created concurrently.  This allows for a small race where cleaning
     // could be done again.
     Atomic::store(&_dead_samples, true);
-    _last_sweep = JfrTicks::now();
+    Atomic::store(&_last_sweep, (int64_t)JfrTicks::now().value());
   }
 }
 
@@ -90,8 +92,8 @@ ObjectSampler::ObjectSampler(size_t size) :
         _total_allocated(0),
         _threshold(0),
         _size(size) {
-  _last_sweep = JfrTicks::now();
   Atomic::store(&_dead_samples, false);
+  Atomic::store(&_last_sweep, (int64_t)JfrTicks::now().value());
 }
 
 ObjectSampler::~ObjectSampler() {
@@ -285,6 +287,6 @@ ObjectSample* ObjectSampler::item_at(int index) {
                                   );
 }
 
-const JfrTicks& ObjectSampler::last_sweep() {
-  return _last_sweep;
+int64_t ObjectSampler::last_sweep() {
+  return Atomic::load(&_last_sweep);
 }

src/hotspot/share/jfr/leakprofiler/sampling/objectSampler.hpp

@@ -26,7 +26,6 @@
 #define SHARE_JFR_LEAKPROFILER_SAMPLING_OBJECTSAMPLER_HPP
 
 #include "memory/allocation.hpp"
-#include "jfr/utilities/jfrTime.hpp"
 
 typedef u8 traceid;
 
@@ -80,7 +79,7 @@ class ObjectSampler : public CHeapObj<mtTracing> {
   // For operations that require exclusive access (non-safepoint)
   static ObjectSampler* acquire();
   static void release();
-  static const JfrTicks& last_sweep();
+  static int64_t last_sweep();
   const ObjectSample* first() const;
   ObjectSample* last() const;
   const ObjectSample* last_resolved() const;