8334215: serviceability/dcmd/thread/PrintMountedVirtualThread.java failing with JTREG_TEST_THREAD_FACTORY=Virtual

Reviewed-by: dholmes

Author: txominpelu

src/hotspot/share/runtime/threads.cpp

@@ -1332,8 +1332,11 @@ void Threads::print_on(outputStream* st, bool print_stacks,
           if (p->is_vthread_mounted()) {
             const oop vt = p->vthread();
             assert(vt != nullptr, "vthread should not be null when vthread is mounted");
-            st->print_cr("   Mounted virtual thread \"%s\" #" INT64_FORMAT, JavaThread::name_for(vt), (int64_t)java_lang_Thread::thread_id(vt));
-            p->print_vthread_stack_on(st);
+            // JavaThread._vthread can refer to the carrier thread. Print only if _vthread refers to a virtual thread.
+            if (vt != thread_oop) {
+              st->print_cr("   Mounted virtual thread #" INT64_FORMAT, (int64_t)java_lang_Thread::thread_id(vt));
+              p->print_vthread_stack_on(st);
+            }
           }
         }
       }

test/hotspot/jtreg/serviceability/dcmd/thread/PrintMountedVirtualThread.java

@@ -26,7 +26,6 @@
 import jdk.test.lib.process.OutputAnalyzer;
 import org.junit.Test;
 
-import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.regex.Pattern;
 
@@ -41,16 +40,18 @@ public class PrintMountedVirtualThread {
 
     public void run(CommandExecutor executor) throws InterruptedException {
         var shouldFinish = new AtomicBoolean(false);
-        var started = new CountDownLatch(1);
+        var started = new AtomicBoolean();
         final Runnable runnable = new DummyRunnable(shouldFinish, started);
         try {
             Thread vthread = Thread.ofVirtual().name("Dummy Vthread").start(runnable);
-            started.await();
+            while (!started.get()) {
+                Thread.sleep(10);
+            }
             /* Execute */
             OutputAnalyzer output = executor.execute("Thread.print");
             output.shouldMatch(".*at " + Pattern.quote(DummyRunnable.class.getName()) + "\\.run.*");
             output.shouldMatch(".*at " + Pattern.quote(DummyRunnable.class.getName()) + "\\.compute.*");
-            output.shouldMatch("Mounted virtual thread " + "\"Dummy Vthread\"" + " #" + vthread.threadId());
+            output.shouldMatch("Mounted virtual thread " + "#" + vthread.threadId());
 
         } finally {
             shouldFinish.set(true);
@@ -63,11 +64,10 @@ public void jmx() throws InterruptedException {
     }
 
     static class DummyRunnable implements Runnable {
-
         private final AtomicBoolean shouldFinish;
-        private final CountDownLatch started;
+        private final AtomicBoolean started;
 
-        public DummyRunnable(AtomicBoolean shouldFinish, CountDownLatch started) {
+        public DummyRunnable(AtomicBoolean shouldFinish, AtomicBoolean started) {
            this.shouldFinish = shouldFinish;
            this.started = started;
         }
@@ -77,12 +77,11 @@ public void run() {
         }
 
         void compute() {
-            started.countDown();
+            started.set(true);
             while (!shouldFinish.get()) {
                 Thread.onSpinWait();
             }
         }
     }
 
-
 }