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