1 /*
2 * Copyright (c) 1997, 2015, 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 #ifndef SHARE_VM_GC_SHARED_GCLOCKER_HPP
26 #define SHARE_VM_GC_SHARED_GCLOCKER_HPP
27
28 #include "gc/shared/collectedHeap.hpp"
29 #include "gc/shared/genCollectedHeap.hpp"
30 #include "memory/universe.hpp"
31 #include "oops/oop.hpp"
32
33 // The direct lock/unlock calls do not force a collection if an unlock
34 // decrements the count to zero. Avoid calling these if at all possible.
35
36 class GC_locker: public AllStatic {
37 private:
38 // The _jni_lock_count keeps track of the number of threads that are
39 // currently in a critical region. It's only kept up to date when
40 // _needs_gc is true. The current value is computed during
41 // safepointing and decremented during the slow path of GC_locker
42 // unlocking.
43 static volatile jint _jni_lock_count; // number of jni active instances.
44 static volatile bool _needs_gc; // heap is filling, we need a GC
45 // note: bool is typedef'd as jint
46 static volatile bool _doing_gc; // unlock_critical() is doing a GC
47
48 #ifdef ASSERT
49 // This lock count is updated for all operations and is used to
50 // validate the jni_lock_count that is computed during safepoints.
51 static volatile jint _debug_jni_lock_count;
52 #endif
53
54 // At a safepoint, visit all threads and count the number of active
55 // critical sections. This is used to ensure that all active
56 // critical sections are exited before a new one is started.
57 static void verify_critical_count() NOT_DEBUG_RETURN;
58
59 static void jni_lock(JavaThread* thread);
60 static void jni_unlock(JavaThread* thread);
61
62 static bool is_active_internal() {
63 verify_critical_count();
64 return _jni_lock_count > 0;
65 }
66
67 public:
68 // Accessors
69 static bool is_active() {
70 assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
71 return is_active_internal();
72 }
73 static bool needs_gc() { return _needs_gc; }
74
75 // Shorthand
76 static bool is_active_and_needs_gc() {
77 // Use is_active_internal since _needs_gc can change from true to
78 // false outside of a safepoint, triggering the assert in
79 // is_active.
80 return needs_gc() && is_active_internal();
81 }
82
83 // In debug mode track the locking state at all times
84 static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN;
85 static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;
86
87 // Set the current lock count
88 static void set_jni_lock_count(int count) {
89 _jni_lock_count = count;
90 verify_critical_count();
91 }
92
93 // Sets _needs_gc if is_active() is true. Returns is_active().
94 static bool check_active_before_gc();
95
96 // Stalls the caller (who should not be in a jni critical section)
97 // until needs_gc() clears. Note however that needs_gc() may be
98 // set at a subsequent safepoint and/or cleared under the
99 // JNICritical_lock, so the caller may not safely assert upon
100 // return from this method that "!needs_gc()" since that is
101 // not a stable predicate.
102 static void stall_until_clear();
103
104 // The following two methods are used for JNI critical regions.
105 // If we find that we failed to perform a GC because the GC_locker
106 // was active, arrange for one as soon as possible by allowing
107 // all threads in critical regions to complete, but not allowing
108 // other critical regions to be entered. The reasons for that are:
109 // 1) a GC request won't be starved by overlapping JNI critical
110 // region activities, which can cause unnecessary OutOfMemory errors.
111 // 2) even if allocation requests can still be satisfied before GC locker
112 // becomes inactive, for example, in tenured generation possibly with
113 // heap expansion, those allocations can trigger lots of safepointing
114 // attempts (ineffective GC attempts) and require Heap_lock which
115 // slow down allocations tremendously.
116 //
117 // Note that critical regions can be nested in a single thread, so
118 // we must allow threads already in critical regions to continue.
119 //
120 // JNI critical regions are the only participants in this scheme
121 // because they are, by spec, well bounded while in a critical region.
122 //
123 // Each of the following two method is split into a fast path and a
124 // slow path. JNICritical_lock is only grabbed in the slow path.
125 // _needs_gc is initially false and every java thread will go
126 // through the fast path, which simply increments or decrements the
127 // current thread's critical count. When GC happens at a safepoint,
128 // GC_locker::is_active() is checked. Since there is no safepoint in
129 // the fast path of lock_critical() and unlock_critical(), there is
130 // no race condition between the fast path and GC. After _needs_gc
131 // is set at a safepoint, every thread will go through the slow path
132 // after the safepoint. Since after a safepoint, each of the
133 // following two methods is either entered from the method entry and
134 // falls into the slow path, or is resumed from the safepoints in
135 // the method, which only exist in the slow path. So when _needs_gc
136 // is set, the slow path is always taken, till _needs_gc is cleared.
137 static void lock_critical(JavaThread* thread);
138 static void unlock_critical(JavaThread* thread);
139
140 static address needs_gc_address() { return (address) &_needs_gc; }
141 };
142
143
144 // A No_GC_Verifier object can be placed in methods where one assumes that
145 // no garbage collection will occur. The destructor will verify this property
146 // unless the constructor is called with argument false (not verifygc).
147 //
148 // The check will only be done in debug mode and if verifygc true.
149
150 class No_GC_Verifier: public StackObj {
151 friend class Pause_No_GC_Verifier;
152
153 protected:
154 bool _verifygc;
155 unsigned int _old_invocations;
156
157 public:
158 #ifdef ASSERT
159 No_GC_Verifier(bool verifygc = true);
160 ~No_GC_Verifier();
161 #else
162 No_GC_Verifier(bool verifygc = true) {}
163 ~No_GC_Verifier() {}
164 #endif
165 };
166
167 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
168 // of a No_GC_Verifier object. If we are not in debug mode or if the
169 // No_GC_Verifier object has a _verifygc value of false, then there
170 // is nothing to do.
171
172 class Pause_No_GC_Verifier: public StackObj {
173 private:
174 No_GC_Verifier * _ngcv;
175
176 public:
177 #ifdef ASSERT
178 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
179 ~Pause_No_GC_Verifier();
180 #else
181 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
182 ~Pause_No_GC_Verifier() {}
183 #endif
184 };
185
186
187 // A No_Safepoint_Verifier object will throw an assertion failure if
188 // the current thread passes a possible safepoint while this object is
189 // instantiated. A safepoint, will either be: an oop allocation, blocking
190 // on a Mutex or JavaLock, or executing a VM operation.
191 //
192 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
193 //
194 class No_Safepoint_Verifier : public No_GC_Verifier {
195 friend class Pause_No_Safepoint_Verifier;
196
197 private:
198 bool _activated;
199 Thread *_thread;
200 public:
201 #ifdef ASSERT
202 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
203 No_GC_Verifier(verifygc),
204 _activated(activated) {
205 _thread = Thread::current();
206 if (_activated) {
207 _thread->_allow_allocation_count++;
208 _thread->_allow_safepoint_count++;
209 }
210 }
211
212 ~No_Safepoint_Verifier() {
213 if (_activated) {
214 _thread->_allow_allocation_count--;
215 _thread->_allow_safepoint_count--;
216 }
217 }
218 #else
219 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
220 ~No_Safepoint_Verifier() {}
221 #endif
222 };
223
224 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
225 // behavior of a No_Safepoint_Verifier object. If we are not in debug
226 // mode then there is nothing to do. If the No_Safepoint_Verifier
227 // object has an _activated value of false, then there is nothing to
228 // do for safepoint and allocation checking, but there may still be
229 // something to do for the underlying No_GC_Verifier object.
230
231 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
232 private:
233 No_Safepoint_Verifier * _nsv;
234
235 public:
236 #ifdef ASSERT
237 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
238 : Pause_No_GC_Verifier(nsv) {
239
240 _nsv = nsv;
241 if (_nsv->_activated) {
242 _nsv->_thread->_allow_allocation_count--;
243 _nsv->_thread->_allow_safepoint_count--;
244 }
245 }
246
247 ~Pause_No_Safepoint_Verifier() {
248 if (_nsv->_activated) {
249 _nsv->_thread->_allow_allocation_count++;
250 _nsv->_thread->_allow_safepoint_count++;
251 }
252 }
253 #else
254 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
255 : Pause_No_GC_Verifier(nsv) {}
256 ~Pause_No_Safepoint_Verifier() {}
257 #endif
258 };
259
260 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
261 // over a section of execution by a thread. Currently, it's used only to
262 // prevent re-entrant calls to GC.
263 class SkipGCALot : public StackObj {
264 private:
265 bool _saved;
266 Thread* _t;
267
268 public:
269 #ifdef ASSERT
270 SkipGCALot(Thread* t) : _t(t) {
271 _saved = _t->skip_gcalot();
272 _t->set_skip_gcalot(true);
273 }
274
275 ~SkipGCALot() {
276 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
277 _t->set_skip_gcalot(_saved);
278 }
279 #else
280 SkipGCALot(Thread* t) { }
281 ~SkipGCALot() { }
282 #endif
283 };
284
285 // JRT_LEAF currently can be called from either _thread_in_Java or
286 // _thread_in_native mode. In _thread_in_native, it is ok
287 // for another thread to trigger GC. The rest of the JRT_LEAF
288 // rules apply.
289 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
290 static bool should_verify_GC();
291 public:
292 #ifdef ASSERT
293 JRT_Leaf_Verifier();
294 ~JRT_Leaf_Verifier();
295 #else
296 JRT_Leaf_Verifier() {}
297 ~JRT_Leaf_Verifier() {}
298 #endif
299 };
300
301 // A No_Alloc_Verifier object can be placed in methods where one assumes that
302 // no allocation will occur. The destructor will verify this property
303 // unless the constructor is called with argument false (not activated).
304 //
305 // The check will only be done in debug mode and if activated.
306 // Note: this only makes sense at safepoints (otherwise, other threads may
307 // allocate concurrently.)
308
309 class No_Alloc_Verifier : public StackObj {
310 private:
311 bool _activated;
312
313 public:
314 #ifdef ASSERT
315 No_Alloc_Verifier(bool activated = true) {
316 _activated = activated;
317 if (_activated) Thread::current()->_allow_allocation_count++;
318 }
319
320 ~No_Alloc_Verifier() {
321 if (_activated) Thread::current()->_allow_allocation_count--;
322 }
323 #else
324 No_Alloc_Verifier(bool activated = true) {}
325 ~No_Alloc_Verifier() {}
326 #endif
327 };
328
329 #endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP