diff -r 2c3cc4b01880 -r c16ac7a2eba4 src/hotspot/share/jfr/leakprofiler/checkpoint/objectSampleCheckpoint.cpp
--- a/src/hotspot/share/jfr/leakprofiler/checkpoint/objectSampleCheckpoint.cpp	Wed Oct 30 16:14:56 2019 +0100
+++ b/src/hotspot/share/jfr/leakprofiler/checkpoint/objectSampleCheckpoint.cpp	Wed Oct 30 19:43:52 2019 +0100
@@ -37,6 +37,7 @@
 #include "jfr/recorder/stacktrace/jfrStackTraceRepository.hpp"
 #include "jfr/utilities/jfrHashtable.hpp"
 #include "jfr/utilities/jfrTypes.hpp"
+#include "runtime/mutexLocker.hpp"
 #include "runtime/safepoint.hpp"
 #include "runtime/thread.hpp"
 #include "utilities/growableArray.hpp"
@@ -271,7 +272,7 @@
   }
   const JfrStackTrace* const stack_trace = resolve(sample);
   DEBUG_ONLY(validate_stack_trace(sample, stack_trace));
-  JfrCheckpointWriter writer(false, true, Thread::current());
+  JfrCheckpointWriter writer;
   writer.write_type(TYPE_STACKTRACE);
   writer.write_count(1);
   ObjectSampleCheckpoint::write_stacktrace(stack_trace, writer);
@@ -291,6 +292,7 @@
 
 // caller needs ResourceMark
 void ObjectSampleCheckpoint::on_rotation(const ObjectSampler* sampler, JfrStackTraceRepository& stack_trace_repo) {
+  assert(JfrStream_lock->owned_by_self(), "invariant");
   assert(sampler != NULL, "invariant");
   assert(LeakProfiler::is_running(), "invariant");
   install_stack_traces(sampler, stack_trace_repo);
@@ -388,7 +390,7 @@
 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();
-  JfrCheckpointWriter writer(false, false, thread);
+  JfrCheckpointWriter writer(thread, false);
   BlobWriter cbw(sampler, writer, last_sweep);
   iterate_samples(cbw, true);
   // reset blob write states
@@ -397,13 +399,14 @@
 }
 
 void ObjectSampleCheckpoint::write(const ObjectSampler* sampler, EdgeStore* edge_store, bool emit_all, Thread* thread) {
+  assert_locked_or_safepoint(JfrStream_lock);
   assert(sampler != NULL, "invariant");
   assert(edge_store != NULL, "invariant");
   assert(thread != NULL, "invariant");
   write_sample_blobs(sampler, emit_all, thread);
   // write reference chains
   if (!edge_store->is_empty()) {
-    JfrCheckpointWriter writer(false, true, thread);
+    JfrCheckpointWriter writer(thread);
     ObjectSampleWriter osw(writer, edge_store);
     edge_store->iterate(osw);
   }