1 /*
2 * Copyright (c) 1997, 2009, 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 // The direct lock/unlock calls do not force a collection if an unlock
26 // decrements the count to zero. Avoid calling these if at all possible.
27
28 class GC_locker: public AllStatic {
29 private:
30 static volatile jint _jni_lock_count; // number of jni active instances
31 static volatile jint _lock_count; // number of other active instances
32 static volatile bool _needs_gc; // heap is filling, we need a GC
33 // note: bool is typedef'd as jint
34 static volatile bool _doing_gc; // unlock_critical() is doing a GC
35
36 // Accessors
37 static bool is_jni_active() {
38 return _jni_lock_count > 0;
39 }
40
41 static void set_needs_gc() {
42 assert(SafepointSynchronize::is_at_safepoint(),
43 "needs_gc is only set at a safepoint");
44 _needs_gc = true;
45 }
46
47 static void clear_needs_gc() {
48 assert_lock_strong(JNICritical_lock);
49 _needs_gc = false;
50 }
51
52 static void jni_lock() {
53 Atomic::inc(&_jni_lock_count);
54 CHECK_UNHANDLED_OOPS_ONLY(
55 if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count++; })
56 assert(Universe::heap() == NULL || !Universe::heap()->is_gc_active(),
57 "locking failed");
58 }
59
60 static void jni_unlock() {
61 Atomic::dec(&_jni_lock_count);
62 CHECK_UNHANDLED_OOPS_ONLY(
63 if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count--; })
64 }
65
66 static void jni_lock_slow();
67 static void jni_unlock_slow();
68
69 public:
70 // Accessors
71 static bool is_active();
72 static bool needs_gc() { return _needs_gc; }
73 // Shorthand
74 static bool is_active_and_needs_gc() { return is_active() && needs_gc();}
75
76 // Calls set_needs_gc() if is_active() is true. Returns is_active().
77 static bool check_active_before_gc();
78
79 // Stalls the caller (who should not be in a jni critical section)
80 // until needs_gc() clears. Note however that needs_gc() may be
81 // set at a subsequent safepoint and/or cleared under the
82 // JNICritical_lock, so the caller may not safely assert upon
83 // return from this method that "!needs_gc()" since that is
84 // not a stable predicate.
85 static void stall_until_clear();
86
87 // Non-structured GC locking: currently needed for JNI. Use with care!
88 static void lock();
89 static void unlock();
90
91 // The following two methods are used for JNI critical regions.
92 // If we find that we failed to perform a GC because the GC_locker
93 // was active, arrange for one as soon as possible by allowing
94 // all threads in critical regions to complete, but not allowing
95 // other critical regions to be entered. The reasons for that are:
96 // 1) a GC request won't be starved by overlapping JNI critical
97 // region activities, which can cause unnecessary OutOfMemory errors.
98 // 2) even if allocation requests can still be satisfied before GC locker
99 // becomes inactive, for example, in tenured generation possibly with
100 // heap expansion, those allocations can trigger lots of safepointing
101 // attempts (ineffective GC attempts) and require Heap_lock which
102 // slow down allocations tremendously.
103 //
104 // Note that critical regions can be nested in a single thread, so
105 // we must allow threads already in critical regions to continue.
106 //
107 // JNI critical regions are the only participants in this scheme
108 // because they are, by spec, well bounded while in a critical region.
109 //
110 // Each of the following two method is split into a fast path and a slow
111 // path. JNICritical_lock is only grabbed in the slow path.
112 // _needs_gc is initially false and every java thread will go
113 // through the fast path (which does the same thing as the slow path
114 // when _needs_gc is false). When GC happens at a safepoint,
115 // GC_locker::is_active() is checked. Since there is no safepoint in the
116 // fast path of lock_critical() and unlock_critical(), there is no race
117 // condition between the fast path and GC. After _needs_gc is set at a
118 // safepoint, every thread will go through the slow path after the safepoint.
119 // Since after a safepoint, each of the following two methods is either
120 // entered from the method entry and falls into the slow path, or is
121 // resumed from the safepoints in the method, which only exist in the slow
122 // path. So when _needs_gc is set, the slow path is always taken, till
123 // _needs_gc is cleared.
124 static void lock_critical(JavaThread* thread);
125 static void unlock_critical(JavaThread* thread);
126 };
127
128
129 // A No_GC_Verifier object can be placed in methods where one assumes that
130 // no garbage collection will occur. The destructor will verify this property
131 // unless the constructor is called with argument false (not verifygc).
132 //
133 // The check will only be done in debug mode and if verifygc true.
134
135 class No_GC_Verifier: public StackObj {
136 friend class Pause_No_GC_Verifier;
137
138 protected:
139 bool _verifygc;
140 unsigned int _old_invocations;
141
142 public:
143 #ifdef ASSERT
144 No_GC_Verifier(bool verifygc = true);
145 ~No_GC_Verifier();
146 #else
147 No_GC_Verifier(bool verifygc = true) {}
148 ~No_GC_Verifier() {}
149 #endif
150 };
151
152 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
153 // of a No_GC_Verifier object. If we are not in debug mode or if the
154 // No_GC_Verifier object has a _verifygc value of false, then there
155 // is nothing to do.
156
157 class Pause_No_GC_Verifier: public StackObj {
158 private:
159 No_GC_Verifier * _ngcv;
160
161 public:
162 #ifdef ASSERT
163 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
164 ~Pause_No_GC_Verifier();
165 #else
166 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
167 ~Pause_No_GC_Verifier() {}
168 #endif
169 };
170
171
172 // A No_Safepoint_Verifier object will throw an assertion failure if
173 // the current thread passes a possible safepoint while this object is
174 // instantiated. A safepoint, will either be: an oop allocation, blocking
175 // on a Mutex or JavaLock, or executing a VM operation.
176 //
177 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
178 //
179 class No_Safepoint_Verifier : public No_GC_Verifier {
180 friend class Pause_No_Safepoint_Verifier;
181
182 private:
183 bool _activated;
184 Thread *_thread;
185 public:
186 #ifdef ASSERT
187 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
188 No_GC_Verifier(verifygc),
189 _activated(activated) {
190 _thread = Thread::current();
191 if (_activated) {
192 _thread->_allow_allocation_count++;
193 _thread->_allow_safepoint_count++;
194 }
195 }
196
197 ~No_Safepoint_Verifier() {
198 if (_activated) {
199 _thread->_allow_allocation_count--;
200 _thread->_allow_safepoint_count--;
201 }
202 }
203 #else
204 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
205 ~No_Safepoint_Verifier() {}
206 #endif
207 };
208
209 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
210 // behavior of a No_Safepoint_Verifier object. If we are not in debug
211 // mode then there is nothing to do. If the No_Safepoint_Verifier
212 // object has an _activated value of false, then there is nothing to
213 // do for safepoint and allocation checking, but there may still be
214 // something to do for the underlying No_GC_Verifier object.
215
216 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
217 private:
218 No_Safepoint_Verifier * _nsv;
219
220 public:
221 #ifdef ASSERT
222 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
223 : Pause_No_GC_Verifier(nsv) {
224
225 _nsv = nsv;
226 if (_nsv->_activated) {
227 _nsv->_thread->_allow_allocation_count--;
228 _nsv->_thread->_allow_safepoint_count--;
229 }
230 }
231
232 ~Pause_No_Safepoint_Verifier() {
233 if (_nsv->_activated) {
234 _nsv->_thread->_allow_allocation_count++;
235 _nsv->_thread->_allow_safepoint_count++;
236 }
237 }
238 #else
239 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
240 : Pause_No_GC_Verifier(nsv) {}
241 ~Pause_No_Safepoint_Verifier() {}
242 #endif
243 };
244
245 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
246 // over a section of execution by a thread. Currently, it's used only to
247 // prevent re-entrant calls to GC.
248 class SkipGCALot : public StackObj {
249 private:
250 bool _saved;
251 Thread* _t;
252
253 public:
254 #ifdef ASSERT
255 SkipGCALot(Thread* t) : _t(t) {
256 _saved = _t->skip_gcalot();
257 _t->set_skip_gcalot(true);
258 }
259
260 ~SkipGCALot() {
261 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
262 _t->set_skip_gcalot(_saved);
263 }
264 #else
265 SkipGCALot(Thread* t) { }
266 ~SkipGCALot() { }
267 #endif
268 };
269
270 // JRT_LEAF currently can be called from either _thread_in_Java or
271 // _thread_in_native mode. In _thread_in_native, it is ok
272 // for another thread to trigger GC. The rest of the JRT_LEAF
273 // rules apply.
274 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
275 static bool should_verify_GC();
276 public:
277 #ifdef ASSERT
278 JRT_Leaf_Verifier();
279 ~JRT_Leaf_Verifier();
280 #else
281 JRT_Leaf_Verifier() {}
282 ~JRT_Leaf_Verifier() {}
283 #endif
284 };
285
286 // A No_Alloc_Verifier object can be placed in methods where one assumes that
287 // no allocation will occur. The destructor will verify this property
288 // unless the constructor is called with argument false (not activated).
289 //
290 // The check will only be done in debug mode and if activated.
291 // Note: this only makes sense at safepoints (otherwise, other threads may
292 // allocate concurrently.)
293
294 class No_Alloc_Verifier : public StackObj {
295 private:
296 bool _activated;
297
298 public:
299 #ifdef ASSERT
300 No_Alloc_Verifier(bool activated = true) {
301 _activated = activated;
302 if (_activated) Thread::current()->_allow_allocation_count++;
303 }
304
305 ~No_Alloc_Verifier() {
306 if (_activated) Thread::current()->_allow_allocation_count--;
307 }
308 #else
309 No_Alloc_Verifier(bool activated = true) {}
310 ~No_Alloc_Verifier() {}
311 #endif
312 };