1 /*
2 * Copyright (c) 1997, 2013, 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.
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23 */
24
25 #ifndef SHARE_VM_MEMORY_GCLOCKER_HPP
26 #define SHARE_VM_MEMORY_GCLOCKER_HPP
27
28 #include "gc_interface/collectedHeap.hpp"
29 #include "memory/genCollectedHeap.hpp"
30 #include "memory/universe.hpp"
31 #include "oops/oop.hpp"
32 #include "runtime/thread.inline.hpp"
33 #ifdef TARGET_OS_FAMILY_linux
34 # include "os_linux.inline.hpp"
35 #endif
36 #ifdef TARGET_OS_FAMILY_solaris
37 # include "os_solaris.inline.hpp"
38 #endif
39 #ifdef TARGET_OS_FAMILY_windows
40 # include "os_windows.inline.hpp"
41 #endif
42 #ifdef TARGET_OS_FAMILY_bsd
43 # include "os_bsd.inline.hpp"
44 #endif
45
46 // The direct lock/unlock calls do not force a collection if an unlock
47 // decrements the count to zero. Avoid calling these if at all possible.
48
49 class GC_locker: public AllStatic {
50 private:
51 // The _jni_lock_count keeps track of the number of threads that are
52 // currently in a critical region. It's only kept up to date when
53 // _needs_gc is true. The current value is computed during
54 // safepointing and decremented during the slow path of GC_locker
55 // unlocking.
56 static volatile jint _jni_lock_count; // number of jni active instances.
57 static volatile bool _needs_gc; // heap is filling, we need a GC
58 // note: bool is typedef'd as jint
59 static volatile bool _doing_gc; // unlock_critical() is doing a GC
60
61 #ifdef ASSERT
62 // This lock count is updated for all operations and is used to
63 // validate the jni_lock_count that is computed during safepoints.
64 static volatile jint _debug_jni_lock_count;
65 #endif
66
67 // At a safepoint, visit all threads and count the number of active
68 // critical sections. This is used to ensure that all active
69 // critical sections are exited before a new one is started.
70 static void verify_critical_count() NOT_DEBUG_RETURN;
71
72 static void jni_lock(JavaThread* thread);
73 static void jni_unlock(JavaThread* thread);
74
75 static bool is_active_internal() {
76 verify_critical_count();
77 return _jni_lock_count > 0;
78 }
79
80 public:
81 // Accessors
82 static bool is_active() {
83 assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
84 return is_active_internal();
85 }
86 static bool needs_gc() { return _needs_gc; }
87
88 // Shorthand
89 static bool is_active_and_needs_gc() {
90 // Use is_active_internal since _needs_gc can change from true to
91 // false outside of a safepoint, triggering the assert in
92 // is_active.
93 return needs_gc() && is_active_internal();
94 }
95
96 // In debug mode track the locking state at all times
97 static void increment_debug_jni_lock_count() {
98 #ifdef ASSERT
99 assert(_debug_jni_lock_count >= 0, "bad value");
100 Atomic::inc(&_debug_jni_lock_count);
101 #endif
102 }
103 static void decrement_debug_jni_lock_count() {
104 #ifdef ASSERT
105 assert(_debug_jni_lock_count > 0, "bad value");
106 Atomic::dec(&_debug_jni_lock_count);
107 #endif
108 }
109
110 // Set the current lock count
111 static void set_jni_lock_count(int count) {
112 _jni_lock_count = count;
113 verify_critical_count();
114 }
115
116 // Sets _needs_gc if is_active() is true. Returns is_active().
117 static bool check_active_before_gc();
118
119 // Stalls the caller (who should not be in a jni critical section)
120 // until needs_gc() clears. Note however that needs_gc() may be
121 // set at a subsequent safepoint and/or cleared under the
122 // JNICritical_lock, so the caller may not safely assert upon
123 // return from this method that "!needs_gc()" since that is
124 // not a stable predicate.
125 static void stall_until_clear();
126
127 // The following two methods are used for JNI critical regions.
128 // If we find that we failed to perform a GC because the GC_locker
129 // was active, arrange for one as soon as possible by allowing
130 // all threads in critical regions to complete, but not allowing
131 // other critical regions to be entered. The reasons for that are:
132 // 1) a GC request won't be starved by overlapping JNI critical
133 // region activities, which can cause unnecessary OutOfMemory errors.
134 // 2) even if allocation requests can still be satisfied before GC locker
135 // becomes inactive, for example, in tenured generation possibly with
136 // heap expansion, those allocations can trigger lots of safepointing
137 // attempts (ineffective GC attempts) and require Heap_lock which
138 // slow down allocations tremendously.
139 //
140 // Note that critical regions can be nested in a single thread, so
141 // we must allow threads already in critical regions to continue.
142 //
143 // JNI critical regions are the only participants in this scheme
144 // because they are, by spec, well bounded while in a critical region.
145 //
146 // Each of the following two method is split into a fast path and a
147 // slow path. JNICritical_lock is only grabbed in the slow path.
148 // _needs_gc is initially false and every java thread will go
149 // through the fast path, which simply increments or decrements the
150 // current thread's critical count. When GC happens at a safepoint,
151 // GC_locker::is_active() is checked. Since there is no safepoint in
152 // the fast path of lock_critical() and unlock_critical(), there is
153 // no race condition between the fast path and GC. After _needs_gc
154 // is set at a safepoint, every thread will go through the slow path
155 // after the safepoint. Since after a safepoint, each of the
156 // following two methods is either entered from the method entry and
157 // falls into the slow path, or is resumed from the safepoints in
158 // the method, which only exist in the slow path. So when _needs_gc
159 // is set, the slow path is always taken, till _needs_gc is cleared.
160 static void lock_critical(JavaThread* thread);
161 static void unlock_critical(JavaThread* thread);
162
163 static address needs_gc_address() { return (address) &_needs_gc; }
164 };
165
166
167 // A No_GC_Verifier object can be placed in methods where one assumes that
168 // no garbage collection will occur. The destructor will verify this property
169 // unless the constructor is called with argument false (not verifygc).
170 //
171 // The check will only be done in debug mode and if verifygc true.
172
173 class No_GC_Verifier: public StackObj {
174 friend class Pause_No_GC_Verifier;
175
176 protected:
177 bool _verifygc;
178 unsigned int _old_invocations;
179
180 public:
181 #ifdef ASSERT
182 No_GC_Verifier(bool verifygc = true);
183 ~No_GC_Verifier();
184 #else
185 No_GC_Verifier(bool verifygc = true) {}
186 ~No_GC_Verifier() {}
187 #endif
188 };
189
190 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
191 // of a No_GC_Verifier object. If we are not in debug mode or if the
192 // No_GC_Verifier object has a _verifygc value of false, then there
193 // is nothing to do.
194
195 class Pause_No_GC_Verifier: public StackObj {
196 private:
197 No_GC_Verifier * _ngcv;
198
199 public:
200 #ifdef ASSERT
201 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
202 ~Pause_No_GC_Verifier();
203 #else
204 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
205 ~Pause_No_GC_Verifier() {}
206 #endif
207 };
208
209
210 // A No_Safepoint_Verifier object will throw an assertion failure if
211 // the current thread passes a possible safepoint while this object is
212 // instantiated. A safepoint, will either be: an oop allocation, blocking
213 // on a Mutex or JavaLock, or executing a VM operation.
214 //
215 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
216 //
217 class No_Safepoint_Verifier : public No_GC_Verifier {
218 friend class Pause_No_Safepoint_Verifier;
219
220 private:
221 bool _activated;
222 Thread *_thread;
223 public:
224 #ifdef ASSERT
225 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
226 No_GC_Verifier(verifygc),
227 _activated(false) {
228 _thread = Thread::current();
229 if (activated) {
230 enable();
231 }
232 }
233
234 void enable() {
235 assert(!_activated, "expected");
236 _thread->_allow_allocation_count++;
237 _thread->_allow_safepoint_count++;
238 _activated = true;
239 }
240
241 void disable() {
242 assert(_activated, "expected");
243 _thread->_allow_allocation_count--;
244 _thread->_allow_safepoint_count--;
245 _activated = false;
246 }
247
248 ~No_Safepoint_Verifier() {
249 if (_activated) {
250 disable();
251 }
252 }
253 #else
254 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
255 void enable() {}
256 void disable() {}
257 ~No_Safepoint_Verifier() {}
258 #endif
259 };
260
261 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
262 // behavior of a No_Safepoint_Verifier object. If we are not in debug
263 // mode then there is nothing to do. If the No_Safepoint_Verifier
264 // object has an _activated value of false, then there is nothing to
265 // do for safepoint and allocation checking, but there may still be
266 // something to do for the underlying No_GC_Verifier object.
267
268 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
269 private:
270 No_Safepoint_Verifier * _nsv;
271
272 public:
273 #ifdef ASSERT
274 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
275 : Pause_No_GC_Verifier(nsv) {
276
277 _nsv = nsv;
278 if (_nsv->_activated) {
279 _nsv->_thread->_allow_allocation_count--;
280 _nsv->_thread->_allow_safepoint_count--;
281 }
282 }
283
284 ~Pause_No_Safepoint_Verifier() {
285 if (_nsv->_activated) {
286 _nsv->_thread->_allow_allocation_count++;
287 _nsv->_thread->_allow_safepoint_count++;
288 }
289 }
290 #else
291 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
292 : Pause_No_GC_Verifier(nsv) {}
293 ~Pause_No_Safepoint_Verifier() {}
294 #endif
295 };
296
297 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
298 // over a section of execution by a thread. Currently, it's used only to
299 // prevent re-entrant calls to GC.
300 class SkipGCALot : public StackObj {
301 private:
302 bool _saved;
303 Thread* _t;
304
305 public:
306 #ifdef ASSERT
307 SkipGCALot(Thread* t) : _t(t) {
308 _saved = _t->skip_gcalot();
309 _t->set_skip_gcalot(true);
310 }
311
312 ~SkipGCALot() {
313 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
314 _t->set_skip_gcalot(_saved);
315 }
316 #else
317 SkipGCALot(Thread* t) { }
318 ~SkipGCALot() { }
319 #endif
320 };
321
322 // JRT_LEAF currently can be called from either _thread_in_Java or
323 // _thread_in_native mode. In _thread_in_native, it is ok
324 // for another thread to trigger GC. The rest of the JRT_LEAF
325 // rules apply.
326 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
327 static bool should_verify_GC();
328 public:
329 #ifdef ASSERT
330 JRT_Leaf_Verifier();
331 ~JRT_Leaf_Verifier();
332 #else
333 JRT_Leaf_Verifier() {}
334 ~JRT_Leaf_Verifier() {}
335 #endif
336 };
337
338 // A No_Alloc_Verifier object can be placed in methods where one assumes that
339 // no allocation will occur. The destructor will verify this property
340 // unless the constructor is called with argument false (not activated).
341 //
342 // The check will only be done in debug mode and if activated.
343 // Note: this only makes sense at safepoints (otherwise, other threads may
344 // allocate concurrently.)
345
346 class No_Alloc_Verifier : public StackObj {
347 private:
348 bool _activated;
349
350 public:
351 #ifdef ASSERT
352 No_Alloc_Verifier(bool activated = true) {
353 _activated = activated;
354 if (_activated) Thread::current()->_allow_allocation_count++;
355 }
356
357 ~No_Alloc_Verifier() {
358 if (_activated) Thread::current()->_allow_allocation_count--;
359 }
360 #else
361 No_Alloc_Verifier(bool activated = true) {}
362 ~No_Alloc_Verifier() {}
363 #endif
364 };
365
366 #endif // SHARE_VM_MEMORY_GCLOCKER_HPP