1 /*
2 * Copyright (c) 1997, 2012, 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 #ifdef TARGET_OS_FAMILY_linux
33 # include "os_linux.inline.hpp"
34 # include "thread_linux.inline.hpp"
35 #endif
36 #ifdef TARGET_OS_FAMILY_solaris
37 # include "os_solaris.inline.hpp"
38 # include "thread_solaris.inline.hpp"
39 #endif
40 #ifdef TARGET_OS_FAMILY_windows
41 # include "os_windows.inline.hpp"
42 # include "thread_windows.inline.hpp"
43 #endif
44 #ifdef TARGET_OS_FAMILY_bsd
45 # include "os_bsd.inline.hpp"
46 # include "thread_bsd.inline.hpp"
47 #endif
48
49 // The direct lock/unlock calls do not force a collection if an unlock
50 // decrements the count to zero. Avoid calling these if at all possible.
51
52 class GC_locker: public AllStatic {
53 private:
54 // The _jni_lock_count keeps track of the number of threads that are
55 // currently in a critical region. It's only kept up to date when
56 // _needs_gc is true. The current value is computed during
57 // safepointing and decremented during the slow path of GC_locker
58 // unlocking.
59 static volatile jint _jni_lock_count; // number of jni active instances.
60
61 static volatile jint _lock_count; // number of other active instances
62 static volatile bool _needs_gc; // heap is filling, we need a GC
63 // note: bool is typedef'd as jint
64 static volatile bool _doing_gc; // unlock_critical() is doing a GC
65
66 #ifdef ASSERT
67 // This lock count is updated for all operations and is used to
68 // validate the jni_lock_count that is computed during safepoints.
69 static volatile jint _debug_jni_lock_count;
70 #endif
71
72 // Accessors
73 static bool is_jni_active() {
74 assert(_needs_gc, "only valid when _needs_gc is set");
75 return _jni_lock_count > 0;
76 }
77
78 // At a safepoint, visit all threads and count the number of active
79 // critical sections. This is used to ensure that all active
80 // critical sections are exited before a new one is started.
81 static void verify_critical_count() NOT_DEBUG_RETURN;
82
83 static void jni_lock(JavaThread* thread);
84 static void jni_unlock(JavaThread* thread);
85
86 static bool is_active_internal() {
87 verify_critical_count();
88 return _lock_count > 0 || _jni_lock_count > 0;
89 }
90
91 public:
92 // Accessors
93 static bool is_active() {
94 assert(_needs_gc || SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
95 return is_active_internal();
96 }
97 static bool needs_gc() { return _needs_gc; }
98
99 // Shorthand
100 static bool is_active_and_needs_gc() {
101 // Use is_active_internal since _needs_gc can change from true to
102 // false outside of a safepoint, triggering the assert in
103 // is_active.
104 return needs_gc() && is_active_internal();
105 }
106
107 // In debug mode track the locking state at all times
108 static void increment_debug_jni_lock_count() {
109 #ifdef ASSERT
110 assert(_debug_jni_lock_count >= 0, "bad value");
111 Atomic::inc(&_debug_jni_lock_count);
112 #endif
113 }
114 static void decrement_debug_jni_lock_count() {
115 #ifdef ASSERT
116 assert(_debug_jni_lock_count > 0, "bad value");
117 Atomic::dec(&_debug_jni_lock_count);
118 #endif
119 }
120
121 // Set the current lock count
122 static void set_jni_lock_count(int count) {
123 _jni_lock_count = count;
124 verify_critical_count();
125 }
126
127 // Sets _needs_gc if is_active() is true. Returns is_active().
128 static bool check_active_before_gc();
129
130 // Stalls the caller (who should not be in a jni critical section)
131 // until needs_gc() clears. Note however that needs_gc() may be
132 // set at a subsequent safepoint and/or cleared under the
133 // JNICritical_lock, so the caller may not safely assert upon
134 // return from this method that "!needs_gc()" since that is
135 // not a stable predicate.
136 static void stall_until_clear();
137
138 // Non-structured GC locking: currently needed for JNI. Use with care!
139 static void lock();
140 static void unlock();
141
142 // The following two methods are used for JNI critical regions.
143 // If we find that we failed to perform a GC because the GC_locker
144 // was active, arrange for one as soon as possible by allowing
145 // all threads in critical regions to complete, but not allowing
146 // other critical regions to be entered. The reasons for that are:
147 // 1) a GC request won't be starved by overlapping JNI critical
148 // region activities, which can cause unnecessary OutOfMemory errors.
149 // 2) even if allocation requests can still be satisfied before GC locker
150 // becomes inactive, for example, in tenured generation possibly with
151 // heap expansion, those allocations can trigger lots of safepointing
152 // attempts (ineffective GC attempts) and require Heap_lock which
153 // slow down allocations tremendously.
154 //
155 // Note that critical regions can be nested in a single thread, so
156 // we must allow threads already in critical regions to continue.
157 //
158 // JNI critical regions are the only participants in this scheme
159 // because they are, by spec, well bounded while in a critical region.
160 //
161 // Each of the following two method is split into a fast path and a
162 // slow path. JNICritical_lock is only grabbed in the slow path.
163 // _needs_gc is initially false and every java thread will go
164 // through the fast path, which simply increments or decrements the
165 // current thread's critical count. When GC happens at a safepoint,
166 // GC_locker::is_active() is checked. Since there is no safepoint in
167 // the fast path of lock_critical() and unlock_critical(), there is
168 // no race condition between the fast path and GC. After _needs_gc
169 // is set at a safepoint, every thread will go through the slow path
170 // after the safepoint. Since after a safepoint, each of the
171 // following two methods is either entered from the method entry and
172 // falls into the slow path, or is resumed from the safepoints in
173 // the method, which only exist in the slow path. So when _needs_gc
174 // is set, the slow path is always taken, till _needs_gc is cleared.
175 static void lock_critical(JavaThread* thread);
176 static void unlock_critical(JavaThread* thread);
177
178 static address needs_gc_address() { return (address) &_needs_gc; }
179 };
180
181
182 // A No_GC_Verifier object can be placed in methods where one assumes that
183 // no garbage collection will occur. The destructor will verify this property
184 // unless the constructor is called with argument false (not verifygc).
185 //
186 // The check will only be done in debug mode and if verifygc true.
187
188 class No_GC_Verifier: public StackObj {
189 friend class Pause_No_GC_Verifier;
190
191 protected:
192 bool _verifygc;
193 unsigned int _old_invocations;
194
195 public:
196 #ifdef ASSERT
197 No_GC_Verifier(bool verifygc = true);
198 ~No_GC_Verifier();
199 #else
200 No_GC_Verifier(bool verifygc = true) {}
201 ~No_GC_Verifier() {}
202 #endif
203 };
204
205 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
206 // of a No_GC_Verifier object. If we are not in debug mode or if the
207 // No_GC_Verifier object has a _verifygc value of false, then there
208 // is nothing to do.
209
210 class Pause_No_GC_Verifier: public StackObj {
211 private:
212 No_GC_Verifier * _ngcv;
213
214 public:
215 #ifdef ASSERT
216 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
217 ~Pause_No_GC_Verifier();
218 #else
219 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
220 ~Pause_No_GC_Verifier() {}
221 #endif
222 };
223
224
225 // A No_Safepoint_Verifier object will throw an assertion failure if
226 // the current thread passes a possible safepoint while this object is
227 // instantiated. A safepoint, will either be: an oop allocation, blocking
228 // on a Mutex or JavaLock, or executing a VM operation.
229 //
230 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
231 //
232 class No_Safepoint_Verifier : public No_GC_Verifier {
233 friend class Pause_No_Safepoint_Verifier;
234
235 private:
236 bool _activated;
237 Thread *_thread;
238 public:
239 #ifdef ASSERT
240 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
241 No_GC_Verifier(verifygc),
242 _activated(activated) {
243 _thread = Thread::current();
244 if (_activated) {
245 _thread->_allow_allocation_count++;
246 _thread->_allow_safepoint_count++;
247 }
248 }
249
250 ~No_Safepoint_Verifier() {
251 if (_activated) {
252 _thread->_allow_allocation_count--;
253 _thread->_allow_safepoint_count--;
254 }
255 }
256 #else
257 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
258 ~No_Safepoint_Verifier() {}
259 #endif
260 };
261
262 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
263 // behavior of a No_Safepoint_Verifier object. If we are not in debug
264 // mode then there is nothing to do. If the No_Safepoint_Verifier
265 // object has an _activated value of false, then there is nothing to
266 // do for safepoint and allocation checking, but there may still be
267 // something to do for the underlying No_GC_Verifier object.
268
269 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
270 private:
271 No_Safepoint_Verifier * _nsv;
272
273 public:
274 #ifdef ASSERT
275 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
276 : Pause_No_GC_Verifier(nsv) {
277
278 _nsv = nsv;
279 if (_nsv->_activated) {
280 _nsv->_thread->_allow_allocation_count--;
281 _nsv->_thread->_allow_safepoint_count--;
282 }
283 }
284
285 ~Pause_No_Safepoint_Verifier() {
286 if (_nsv->_activated) {
287 _nsv->_thread->_allow_allocation_count++;
288 _nsv->_thread->_allow_safepoint_count++;
289 }
290 }
291 #else
292 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
293 : Pause_No_GC_Verifier(nsv) {}
294 ~Pause_No_Safepoint_Verifier() {}
295 #endif
296 };
297
298 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
299 // over a section of execution by a thread. Currently, it's used only to
300 // prevent re-entrant calls to GC.
301 class SkipGCALot : public StackObj {
302 private:
303 bool _saved;
304 Thread* _t;
305
306 public:
307 #ifdef ASSERT
308 SkipGCALot(Thread* t) : _t(t) {
309 _saved = _t->skip_gcalot();
310 _t->set_skip_gcalot(true);
311 }
312
313 ~SkipGCALot() {
314 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
315 _t->set_skip_gcalot(_saved);
316 }
317 #else
318 SkipGCALot(Thread* t) { }
319 ~SkipGCALot() { }
320 #endif
321 };
322
323 // JRT_LEAF currently can be called from either _thread_in_Java or
324 // _thread_in_native mode. In _thread_in_native, it is ok
325 // for another thread to trigger GC. The rest of the JRT_LEAF
326 // rules apply.
327 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
328 static bool should_verify_GC();
329 public:
330 #ifdef ASSERT
331 JRT_Leaf_Verifier();
332 ~JRT_Leaf_Verifier();
333 #else
334 JRT_Leaf_Verifier() {}
335 ~JRT_Leaf_Verifier() {}
336 #endif
337 };
338
339 // A No_Alloc_Verifier object can be placed in methods where one assumes that
340 // no allocation will occur. The destructor will verify this property
341 // unless the constructor is called with argument false (not activated).
342 //
343 // The check will only be done in debug mode and if activated.
344 // Note: this only makes sense at safepoints (otherwise, other threads may
345 // allocate concurrently.)
346
347 class No_Alloc_Verifier : public StackObj {
348 private:
349 bool _activated;
350
351 public:
352 #ifdef ASSERT
353 No_Alloc_Verifier(bool activated = true) {
354 _activated = activated;
355 if (_activated) Thread::current()->_allow_allocation_count++;
356 }
357
358 ~No_Alloc_Verifier() {
359 if (_activated) Thread::current()->_allow_allocation_count--;
360 }
361 #else
362 No_Alloc_Verifier(bool activated = true) {}
363 ~No_Alloc_Verifier() {}
364 #endif
365 };
366
367 #endif // SHARE_VM_MEMORY_GCLOCKER_HPP