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.
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_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() NOT_DEBUG_RETURN;
98 static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;
99
100 // Set the current lock count
101 static void set_jni_lock_count(int count) {
102 _jni_lock_count = count;
103 verify_critical_count();
104 }
105
106 // Sets _needs_gc if is_active() is true. Returns is_active().
107 static bool check_active_before_gc();
108
109 // Stalls the caller (who should not be in a jni critical section)
110 // until needs_gc() clears. Note however that needs_gc() may be
111 // set at a subsequent safepoint and/or cleared under the
112 // JNICritical_lock, so the caller may not safely assert upon
113 // return from this method that "!needs_gc()" since that is
114 // not a stable predicate.
115 static void stall_until_clear();
116
117 // The following two methods are used for JNI critical regions.
118 // If we find that we failed to perform a GC because the GC_locker
119 // was active, arrange for one as soon as possible by allowing
120 // all threads in critical regions to complete, but not allowing
121 // other critical regions to be entered. The reasons for that are:
122 // 1) a GC request won't be starved by overlapping JNI critical
123 // region activities, which can cause unnecessary OutOfMemory errors.
124 // 2) even if allocation requests can still be satisfied before GC locker
125 // becomes inactive, for example, in tenured generation possibly with
126 // heap expansion, those allocations can trigger lots of safepointing
127 // attempts (ineffective GC attempts) and require Heap_lock which
128 // slow down allocations tremendously.
129 //
130 // Note that critical regions can be nested in a single thread, so
131 // we must allow threads already in critical regions to continue.
132 //
133 // JNI critical regions are the only participants in this scheme
134 // because they are, by spec, well bounded while in a critical region.
135 //
136 // Each of the following two method is split into a fast path and a
137 // slow path. JNICritical_lock is only grabbed in the slow path.
138 // _needs_gc is initially false and every java thread will go
139 // through the fast path, which simply increments or decrements the
140 // current thread's critical count. When GC happens at a safepoint,
141 // GC_locker::is_active() is checked. Since there is no safepoint in
142 // the fast path of lock_critical() and unlock_critical(), there is
143 // no race condition between the fast path and GC. After _needs_gc
144 // is set at a safepoint, every thread will go through the slow path
145 // after the safepoint. Since after a safepoint, each of the
146 // following two methods is either entered from the method entry and
147 // falls into the slow path, or is resumed from the safepoints in
148 // the method, which only exist in the slow path. So when _needs_gc
149 // is set, the slow path is always taken, till _needs_gc is cleared.
150 static void lock_critical(JavaThread* thread);
151 static void unlock_critical(JavaThread* thread);
152
153 static address needs_gc_address() { return (address) &_needs_gc; }
154 };
155
156
157 // A No_GC_Verifier object can be placed in methods where one assumes that
158 // no garbage collection will occur. The destructor will verify this property
159 // unless the constructor is called with argument false (not verifygc).
160 //
161 // The check will only be done in debug mode and if verifygc true.
162
163 class No_GC_Verifier: public StackObj {
164 friend class Pause_No_GC_Verifier;
165
166 protected:
167 bool _verifygc;
168 unsigned int _old_invocations;
169
170 public:
171 #ifdef ASSERT
172 No_GC_Verifier(bool verifygc = true);
173 ~No_GC_Verifier();
174 #else
175 No_GC_Verifier(bool verifygc = true) {}
176 ~No_GC_Verifier() {}
177 #endif
178 };
179
180 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
181 // of a No_GC_Verifier object. If we are not in debug mode or if the
182 // No_GC_Verifier object has a _verifygc value of false, then there
183 // is nothing to do.
184
185 class Pause_No_GC_Verifier: public StackObj {
186 private:
187 No_GC_Verifier * _ngcv;
188
189 public:
190 #ifdef ASSERT
191 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
192 ~Pause_No_GC_Verifier();
193 #else
194 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
195 ~Pause_No_GC_Verifier() {}
196 #endif
197 };
198
199
200 // A No_Safepoint_Verifier object will throw an assertion failure if
201 // the current thread passes a possible safepoint while this object is
202 // instantiated. A safepoint, will either be: an oop allocation, blocking
203 // on a Mutex or JavaLock, or executing a VM operation.
204 //
205 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
206 //
207 class No_Safepoint_Verifier : public No_GC_Verifier {
208 friend class Pause_No_Safepoint_Verifier;
209
210 private:
211 bool _activated;
212 Thread *_thread;
213 public:
214 #ifdef ASSERT
215 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
216 No_GC_Verifier(verifygc),
217 _activated(activated) {
218 _thread = Thread::current();
219 if (_activated) {
220 _thread->_allow_allocation_count++;
221 _thread->_allow_safepoint_count++;
222 }
223 }
224
225 ~No_Safepoint_Verifier() {
226 if (_activated) {
227 _thread->_allow_allocation_count--;
228 _thread->_allow_safepoint_count--;
229 }
230 }
231 #else
232 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
233 ~No_Safepoint_Verifier() {}
234 #endif
235 };
236
237 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
238 // behavior of a No_Safepoint_Verifier object. If we are not in debug
239 // mode then there is nothing to do. If the No_Safepoint_Verifier
240 // object has an _activated value of false, then there is nothing to
241 // do for safepoint and allocation checking, but there may still be
242 // something to do for the underlying No_GC_Verifier object.
243
244 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
245 private:
246 No_Safepoint_Verifier * _nsv;
247
248 public:
249 #ifdef ASSERT
250 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
251 : Pause_No_GC_Verifier(nsv) {
252
253 _nsv = nsv;
254 if (_nsv->_activated) {
255 _nsv->_thread->_allow_allocation_count--;
256 _nsv->_thread->_allow_safepoint_count--;
257 }
258 }
259
260 ~Pause_No_Safepoint_Verifier() {
261 if (_nsv->_activated) {
262 _nsv->_thread->_allow_allocation_count++;
263 _nsv->_thread->_allow_safepoint_count++;
264 }
265 }
266 #else
267 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
268 : Pause_No_GC_Verifier(nsv) {}
269 ~Pause_No_Safepoint_Verifier() {}
270 #endif
271 };
272
273 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
274 // over a section of execution by a thread. Currently, it's used only to
275 // prevent re-entrant calls to GC.
276 class SkipGCALot : public StackObj {
277 private:
278 bool _saved;
279 Thread* _t;
280
281 public:
282 #ifdef ASSERT
283 SkipGCALot(Thread* t) : _t(t) {
284 _saved = _t->skip_gcalot();
285 _t->set_skip_gcalot(true);
286 }
287
288 ~SkipGCALot() {
289 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
290 _t->set_skip_gcalot(_saved);
291 }
292 #else
293 SkipGCALot(Thread* t) { }
294 ~SkipGCALot() { }
295 #endif
296 };
297
298 // JRT_LEAF currently can be called from either _thread_in_Java or
299 // _thread_in_native mode. In _thread_in_native, it is ok
300 // for another thread to trigger GC. The rest of the JRT_LEAF
301 // rules apply.
302 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
303 static bool should_verify_GC();
304 public:
305 #ifdef ASSERT
306 JRT_Leaf_Verifier();
307 ~JRT_Leaf_Verifier();
308 #else
309 JRT_Leaf_Verifier() {}
310 ~JRT_Leaf_Verifier() {}
311 #endif
312 };
313
314 // A No_Alloc_Verifier object can be placed in methods where one assumes that
315 // no allocation will occur. The destructor will verify this property
316 // unless the constructor is called with argument false (not activated).
317 //
318 // The check will only be done in debug mode and if activated.
319 // Note: this only makes sense at safepoints (otherwise, other threads may
320 // allocate concurrently.)
321
322 class No_Alloc_Verifier : public StackObj {
323 private:
324 bool _activated;
325
326 public:
327 #ifdef ASSERT
328 No_Alloc_Verifier(bool activated = true) {
329 _activated = activated;
330 if (_activated) Thread::current()->_allow_allocation_count++;
331 }
332
333 ~No_Alloc_Verifier() {
334 if (_activated) Thread::current()->_allow_allocation_count--;
335 }
336 #else
337 No_Alloc_Verifier(bool activated = true) {}
338 ~No_Alloc_Verifier() {}
339 #endif
340 };
341
342 #endif // SHARE_VM_MEMORY_GCLOCKER_HPP