8305425: Thread.isAlive0 doesn't need to call into the VM

Co-authored-by: Aleksey Shipilev <shade@openjdk.org>
Reviewed-by: shade, coleenp, alanb

Author: David Holmes

make/data/hotspot-symbols/symbols-unix

@@ -162,7 +162,6 @@ JVM_IsRecord
 JVM_IsSameClassPackage
 JVM_IsSharingEnabled
 JVM_IsSupportedJNIVersion
-JVM_IsThreadAlive
 JVM_IsVMGeneratedMethodIx
 JVM_LatestUserDefinedLoader
 JVM_LoadZipLibrary

src/hotspot/share/include/jvm.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2023, 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
@@ -266,9 +266,6 @@ JVM_SetStackWalkContinuation(JNIEnv *env, jobject stackStream, jlong anchor, job
 JNIEXPORT void JNICALL
 JVM_StartThread(JNIEnv *env, jobject thread);
 
-JNIEXPORT jboolean JNICALL
-JVM_IsThreadAlive(JNIEnv *env, jobject thread);
-
 JNIEXPORT void JNICALL
 JVM_SetThreadPriority(JNIEnv *env, jobject thread, jint prio);
 

src/hotspot/share/prims/jvm.cpp

@@ -3020,12 +3020,6 @@ JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
 JVM_END
 
 
-JVM_ENTRY(jboolean, JVM_IsThreadAlive(JNIEnv* env, jobject jthread))
-  oop thread_oop = JNIHandles::resolve_non_null(jthread);
-  return java_lang_Thread::is_alive(thread_oop);
-JVM_END
-
-
 JVM_ENTRY(void, JVM_SetThreadPriority(JNIEnv* env, jobject jthread, jint prio))
   ThreadsListHandle tlh(thread);
   oop java_thread = nullptr;

src/hotspot/share/runtime/javaThread.cpp

@@ -739,7 +739,9 @@ static void ensure_join(JavaThread* thread) {
   // Thread is exiting. So set thread_status field in  java.lang.Thread class to TERMINATED.
   java_lang_Thread::set_thread_status(threadObj(), JavaThreadStatus::TERMINATED);
   // Clear the native thread instance - this makes isAlive return false and allows the join()
-  // to complete once we've done the notify_all below
+  // to complete once we've done the notify_all below. Needs a release() to obey Java Memory Model
+  // requirements.
+  OrderAccess::release();
   java_lang_Thread::set_thread(threadObj(), nullptr);
   lock.notify_all(thread);
   // Ignore pending exception, since we are exiting anyway

src/java.base/share/classes/java/lang/Thread.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2023, 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
@@ -226,8 +226,15 @@ public class Thread implements Runnable {
         registerNatives();
     }
 
-    /* Reserved for exclusive use by the JVM, maybe move to FieldHolder */
-    private long eetop;
+    /*
+     * Reserved for exclusive use by the JVM. Cannot be moved to the FieldHolder
+     * as it needs to be set by the VM for JNI attaching threads, before executing
+     * the constructor that will create the FieldHolder. The historically named
+     * `eetop` holds the address of the underlying VM JavaThread, and is set to
+     * non-zero when the thread is started, and reset to zero when the thread terminates.
+     * A non-zero value indicates this thread isAlive().
+     */
+    private volatile long eetop;
 
     // thread id
     private final long tid;
@@ -1840,9 +1847,8 @@ public final boolean isAlive() {
      * This method is non-final so it can be overridden.
      */
     boolean alive() {
-        return isAlive0();
+        return eetop != 0;
     }
-    private native boolean isAlive0();
 
     /**
      * Throws {@code UnsupportedOperationException}.

src/java.base/share/native/libjava/Thread.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1994, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1994, 2023, 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
@@ -37,7 +37,6 @@
 
 static JNINativeMethod methods[] = {
     {"start0",           "()V",        (void *)&JVM_StartThread},
-    {"isAlive0",         "()Z",        (void *)&JVM_IsThreadAlive},
     {"setPriority0",     "(I)V",       (void *)&JVM_SetThreadPriority},
     {"yield0",           "()V",        (void *)&JVM_Yield},
     {"sleep0",           "(J)V",       (void *)&JVM_Sleep},

test/jdk/java/lang/Thread/IsAlive.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2023, Amazon.com Inc. 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug     8305425
+ * @summary Check Thread.isAlive
+ * @run main/othervm/timeout=10 IsAlive
+ */
+
+public class IsAlive {
+
+    static boolean spinnerDone;
+
+    private static void spin() {
+        try {
+            while (!Thread.currentThread().isInterrupted()) {
+                Thread.sleep(100);
+            }
+        } catch (InterruptedException ie) {
+            // Do nothing, just exit
+        }
+        spinnerDone = true;
+    }
+
+    static volatile boolean checkerReady;
+
+    private static void check(Thread t) {
+        while (!t.isAlive()) {
+            // Burn hard, without any sleeps.
+            // Check that we discover the thread is alive eventually.
+        }
+
+        checkerReady = true;
+
+        while (t.isAlive()) {
+            // Burn hard, without any sleeps.
+            // Check that we discover the thread is not alive eventually.
+        }
+
+        if (!spinnerDone) {
+            throw new RuntimeException("Last write of terminated thread was not seen!");
+        }
+    }
+
+    private static void assertAlive(Thread t) {
+        if (!t.isAlive()) {
+            throw new IllegalStateException("Thread " + t + " is not alive, but it should be");
+        }
+    }
+
+    private static void assertNotAlive(Thread t) {
+        if (t.isAlive()) {
+            throw new IllegalStateException("Thread " + t + " is alive, but it should not be");
+        }
+    }
+
+    public static void main(String args[]) throws Exception {
+        Thread spinner = new Thread(IsAlive::spin);
+        spinner.setName("Spinner");
+        spinner.setDaemon(true);
+
+        Thread checker = new Thread(() -> check(spinner));
+        checker.setName("Checker");
+        checker.setDaemon(true);
+
+        assertNotAlive(spinner);
+        assertNotAlive(checker);
+
+        System.out.println("Starting spinner");
+        spinner.start();
+        assertAlive(spinner);
+
+        System.out.println("Starting checker");
+        checker.start();
+        assertAlive(checker);
+
+        System.out.println("Waiting for checker to catch up");
+        while (!checkerReady) {
+            Thread.sleep(100);
+        }
+
+        System.out.println("Interrupting and joining spinner");
+        spinner.interrupt();
+        spinner.join();
+        assertNotAlive(spinner);
+
+        System.out.println("Joining checker");
+        checker.join();
+        assertNotAlive(checker);
+
+        System.out.println("Complete");
+    }
+}