1 /*
2 * Copyright (c) 2019, 2022, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package jdk.internal.misc;
27
28 import java.util.concurrent.ForkJoinPool;
29 import jdk.internal.access.JavaLangAccess;
30 import jdk.internal.access.JavaUtilConcurrentFJPAccess;
31 import jdk.internal.access.SharedSecrets;
32
33 /**
34 * Defines static methods to mark the beginning and end of a possibly blocking
35 * operation. The methods are intended to be used with try-finally as follows:
36 * {@snippet lang=java :
37 * long comp = Blocker.begin();
38 * try {
39 * // blocking operation
40 * } finally {
41 * Blocker.end(comp);
42 * }
43 * }
44 * If invoked from a virtual thread and the underlying carrier thread is a
45 * CarrierThread then the code in the block runs as if it were in run in
46 * ForkJoinPool.ManagedBlocker. This means the pool can be expanded to support
47 * additional parallelism during the blocking operation.
48 */
49 public class Blocker {
50 private static final JavaLangAccess JLA;
51 static {
52 JLA = SharedSecrets.getJavaLangAccess();
53 if (JLA == null) {
54 throw new InternalError("JavaLangAccess not setup");
55 }
56 }
57
58 private Blocker() { }
59
60 private static Thread currentCarrierThread() {
61 return JLA.currentCarrierThread();
62 }
63
64 /**
65 * Marks the beginning of a possibly blocking operation.
66 * @return the return value from the attempt to compensate or -1 if not attempted
67 */
68 public static long begin() {
69 if (VM.isBooted()
70 && Thread.currentThread().isVirtual()
71 && currentCarrierThread() instanceof CarrierThread ct && !ct.inBlocking()) {
72 ct.beginBlocking();
73 boolean completed = false;
74 try {
75 long comp = ForkJoinPools.beginCompensatedBlock(ct.getPool());
76 assert currentCarrierThread() == ct;
77 completed = true;
78 return comp;
79 } finally {
80 if (!completed) {
81 ct.endBlocking();
82 }
83 }
84 }
85 return -1;
86 }
87
88 /**
89 * Marks the beginning of a possibly blocking operation.
90 * @param blocking true if the operation may block, otherwise false
91 * @return the return value from the attempt to compensate, -1 if not attempted
92 * or blocking is false
93 */
94 public static long begin(boolean blocking) {
95 return (blocking) ? begin() : -1;
96 }
97
98 /**
99 * Marks the end of an operation that may have blocked.
100 * @param compensateReturn the value returned by the begin method
101 */
102 public static void end(long compensateReturn) {
103 if (compensateReturn >= 0) {
104 assert currentCarrierThread() instanceof CarrierThread ct && ct.inBlocking();
105 CarrierThread ct = (CarrierThread) currentCarrierThread();
106 ForkJoinPools.endCompensatedBlock(ct.getPool(), compensateReturn);
107 ct.endBlocking();
108 }
109 }
110
111 /**
112 * Defines static methods to invoke non-public ForkJoinPool methods via the
113 * shared secret support.
114 */
115 private static class ForkJoinPools {
116 private static final JavaUtilConcurrentFJPAccess FJP_ACCESS =
117 SharedSecrets.getJavaUtilConcurrentFJPAccess();
118 static long beginCompensatedBlock(ForkJoinPool pool) {
119 return FJP_ACCESS.beginCompensatedBlock(pool);
120 }
121 static void endCompensatedBlock(ForkJoinPool pool, long post) {
122 FJP_ACCESS.endCompensatedBlock(pool, post);
123 }
124 }
125
126 }