1 /*
2 * Copyright (c) 1997, 2019, 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.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "gc/shared/collectedHeap.hpp"
27 #include "gc/shared/gcLocker.hpp"
28 #include "memory/resourceArea.hpp"
29 #include "memory/universe.hpp"
30 #include "logging/log.hpp"
31 #include "runtime/atomic.hpp"
32 #include "runtime/mutexLocker.inline.hpp"
33 #include "runtime/safepoint.hpp"
34 #include "runtime/thread.inline.hpp"
35 #include "runtime/threadSMR.hpp"
36
37 volatile jint GCLocker::_jni_lock_count = 0;
38 volatile bool GCLocker::_needs_gc = false;
39 volatile bool GCLocker::_doing_gc = false;
40 unsigned int GCLocker::_total_collections = 0;
41
42 #ifdef ASSERT
43 volatile jint GCLocker::_debug_jni_lock_count = 0;
44 #endif
45
46
47 #ifdef ASSERT
48 void GCLocker::verify_critical_count() {
49 if (SafepointSynchronize::is_at_safepoint()) {
50 assert(!needs_gc() || _debug_jni_lock_count == _jni_lock_count, "must agree");
51 int count = 0;
52 // Count the number of threads with critical operations in progress
53 JavaThreadIteratorWithHandle jtiwh;
54 for (; JavaThread *thr = jtiwh.next(); ) {
55 if (thr->in_critical()) {
56 count++;
57 }
58 }
59 if (_jni_lock_count != count) {
60 log_error(gc, verify)("critical counts don't match: %d != %d", _jni_lock_count, count);
61 jtiwh.rewind();
62 for (; JavaThread *thr = jtiwh.next(); ) {
63 if (thr->in_critical()) {
64 log_error(gc, verify)(INTPTR_FORMAT " in_critical %d", p2i(thr), thr->in_critical());
65 }
66 }
67 }
68 assert(_jni_lock_count == count, "must be equal");
69 }
70 }
71
72 // In debug mode track the locking state at all times
73 void GCLocker::increment_debug_jni_lock_count() {
74 assert(_debug_jni_lock_count >= 0, "bad value");
75 Atomic::inc(&_debug_jni_lock_count);
76 }
77
78 void GCLocker::decrement_debug_jni_lock_count() {
79 assert(_debug_jni_lock_count > 0, "bad value");
80 Atomic::dec(&_debug_jni_lock_count);
81 }
82 #endif
83
84 void GCLocker::log_debug_jni(const char* msg) {
85 Log(gc, jni) log;
86 if (log.is_debug()) {
87 ResourceMark rm; // JavaThread::name() allocates to convert to UTF8
88 log.debug("%s Thread \"%s\" %d locked.", msg, Thread::current()->name(), _jni_lock_count);
89 }
90 }
91
92 bool GCLocker::is_at_safepoint() {
93 return SafepointSynchronize::is_at_safepoint();
94 }
95
96 bool GCLocker::check_active_before_gc() {
97 assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
98 if (is_active() && !_needs_gc) {
99 verify_critical_count();
100 _needs_gc = true;
101 log_debug_jni("Setting _needs_gc.");
102 }
103 return is_active();
104 }
105
106 void GCLocker::stall_until_clear() {
107 assert(!JavaThread::current()->in_critical(), "Would deadlock");
108 MonitorLocker ml(JNICritical_lock);
109
110 if (needs_gc()) {
111 log_debug_jni("Allocation failed. Thread stalled by JNI critical section.");
112 }
113
114 // Wait for _needs_gc to be cleared
115 while (needs_gc()) {
116 ml.wait();
117 }
118 }
119
120 bool GCLocker::should_discard(GCCause::Cause cause, uint total_collections) {
121 return (cause == GCCause::_gc_locker) &&
122 (_total_collections != total_collections);
123 }
124
125 void GCLocker::jni_lock(JavaThread* thread) {
126 assert(!thread->in_critical(), "shouldn't currently be in a critical region");
127 MonitorLocker ml(JNICritical_lock);
128 // Block entering threads if we know at least one thread is in a
129 // JNI critical region and we need a GC.
130 // We check that at least one thread is in a critical region before
131 // blocking because blocked threads are woken up by a thread exiting
132 // a JNI critical region.
133 while (is_active_and_needs_gc() || _doing_gc) {
134 ml.wait();
135 }
136 thread->enter_critical();
137 _jni_lock_count++;
138 increment_debug_jni_lock_count();
139 }
140
141 void GCLocker::jni_unlock(JavaThread* thread) {
142 assert(thread->in_last_critical(), "should be exiting critical region");
143 MutexLocker mu(JNICritical_lock);
144 _jni_lock_count--;
145 decrement_debug_jni_lock_count();
146 thread->exit_critical();
147 if (needs_gc() && !is_active_internal()) {
148 // We're the last thread out. Request a GC.
149 // Capture the current total collections, to allow detection of
150 // other collections that make this one unnecessary. The value of
151 // total_collections() is only changed at a safepoint, so there
152 // must not be a safepoint between the lock becoming inactive and
153 // getting the count, else there may be unnecessary GCLocker GCs.
154 _total_collections = Universe::heap()->total_collections();
155 _doing_gc = true;
156 {
157 // Must give up the lock while at a safepoint
158 MutexUnlocker munlock(JNICritical_lock);
159 log_debug_jni("Performing GC after exiting critical section.");
160 Universe::heap()->collect(GCCause::_gc_locker);
161 }
162 _doing_gc = false;
163 _needs_gc = false;
164 JNICritical_lock->notify_all();
165 }
166 }