1 /*
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   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.
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   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).
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  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  20  * or visit www.oracle.com if you need additional information or have any
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  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 GCLocker: 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 GCLocker
  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   static void log_debug_jni(const char* msg);
  68  public:
  69   // Accessors
  70   static bool is_active() {
  71     assert(SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
  72     return is_active_internal();
  73   }
  74   static bool needs_gc()       { return _needs_gc;                        }
  75 
  76   // Shorthand
  77   static bool is_active_and_needs_gc() {
  78     // Use is_active_internal since _needs_gc can change from true to
  79     // false outside of a safepoint, triggering the assert in
  80     // is_active.
  81     return needs_gc() && is_active_internal();
  82   }
  83 
  84   // In debug mode track the locking state at all times
  85   static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN;
  86   static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;
  87 
  88   // Set the current lock count
  89   static void set_jni_lock_count(int count) {
  90     _jni_lock_count = count;
  91     verify_critical_count();
  92   }
  93 
  94   // Sets _needs_gc if is_active() is true. Returns is_active().
  95   static bool check_active_before_gc();
  96 
  97   // Stalls the caller (who should not be in a jni critical section)
  98   // until needs_gc() clears. Note however that needs_gc() may be
  99   // set at a subsequent safepoint and/or cleared under the
 100   // JNICritical_lock, so the caller may not safely assert upon
 101   // return from this method that "!needs_gc()" since that is
 102   // not a stable predicate.
 103   static void stall_until_clear();
 104 
 105   // The following two methods are used for JNI critical regions.
 106   // If we find that we failed to perform a GC because the GCLocker
 107   // was active, arrange for one as soon as possible by allowing
 108   // all threads in critical regions to complete, but not allowing
 109   // other critical regions to be entered. The reasons for that are:
 110   // 1) a GC request won't be starved by overlapping JNI critical
 111   //    region activities, which can cause unnecessary OutOfMemory errors.
 112   // 2) even if allocation requests can still be satisfied before GC locker
 113   //    becomes inactive, for example, in tenured generation possibly with
 114   //    heap expansion, those allocations can trigger lots of safepointing
 115   //    attempts (ineffective GC attempts) and require Heap_lock which
 116   //    slow down allocations tremendously.
 117   //
 118   // Note that critical regions can be nested in a single thread, so
 119   // we must allow threads already in critical regions to continue.
 120   //
 121   // JNI critical regions are the only participants in this scheme
 122   // because they are, by spec, well bounded while in a critical region.
 123   //
 124   // Each of the following two method is split into a fast path and a
 125   // slow path. JNICritical_lock is only grabbed in the slow path.
 126   // _needs_gc is initially false and every java thread will go
 127   // through the fast path, which simply increments or decrements the
 128   // current thread's critical count.  When GC happens at a safepoint,
 129   // GCLocker::is_active() is checked. Since there is no safepoint in
 130   // the fast path of lock_critical() and unlock_critical(), there is
 131   // no race condition between the fast path and GC. After _needs_gc
 132   // is set at a safepoint, every thread will go through the slow path
 133   // after the safepoint.  Since after a safepoint, each of the
 134   // following two methods is either entered from the method entry and
 135   // falls into the slow path, or is resumed from the safepoints in
 136   // the method, which only exist in the slow path. So when _needs_gc
 137   // is set, the slow path is always taken, till _needs_gc is cleared.
 138   static void lock_critical(JavaThread* thread);
 139   static void unlock_critical(JavaThread* thread);
 140 
 141   static address needs_gc_address() { return (address) &_needs_gc; }
 142 };
 143 
 144 
 145 // A NoGCVerifier object can be placed in methods where one assumes that
 146 // no garbage collection will occur. The destructor will verify this property
 147 // unless the constructor is called with argument false (not verifygc).
 148 //
 149 // The check will only be done in debug mode and if verifygc true.
 150 
 151 class NoGCVerifier: public StackObj {
 152  friend class PauseNoGCVerifier;
 153 
 154  protected:
 155   bool _verifygc;
 156   unsigned int _old_invocations;
 157 
 158  public:
 159 #ifdef ASSERT
 160   NoGCVerifier(bool verifygc = true);
 161   ~NoGCVerifier();
 162 #else
 163   NoGCVerifier(bool verifygc = true) {}
 164   ~NoGCVerifier() {}
 165 #endif
 166 };
 167 
 168 // A PauseNoGCVerifier is used to temporarily pause the behavior
 169 // of a NoGCVerifier object. If we are not in debug mode or if the
 170 // NoGCVerifier object has a _verifygc value of false, then there
 171 // is nothing to do.
 172 
 173 class PauseNoGCVerifier: public StackObj {
 174  private:
 175   NoGCVerifier * _ngcv;
 176 
 177  public:
 178 #ifdef ASSERT
 179   PauseNoGCVerifier(NoGCVerifier * ngcv);
 180   ~PauseNoGCVerifier();
 181 #else
 182   PauseNoGCVerifier(NoGCVerifier * ngcv) {}
 183   ~PauseNoGCVerifier() {}
 184 #endif
 185 };
 186 
 187 
 188 // A NoSafepointVerifier object will throw an assertion failure if
 189 // the current thread passes a possible safepoint while this object is
 190 // instantiated. A safepoint, will either be: an oop allocation, blocking
 191 // on a Mutex or JavaLock, or executing a VM operation.
 192 //
 193 // If StrictSafepointChecks is turned off, it degrades into a NoGCVerifier
 194 //
 195 class NoSafepointVerifier : public NoGCVerifier {
 196  friend class PauseNoSafepointVerifier;
 197 
 198  private:
 199   bool _activated;
 200   Thread *_thread;
 201  public:
 202 #ifdef ASSERT
 203   NoSafepointVerifier(bool activated = true, bool verifygc = true ) :
 204     NoGCVerifier(verifygc),
 205     _activated(activated) {
 206     _thread = Thread::current();
 207     if (_activated) {
 208       _thread->_allow_allocation_count++;
 209       _thread->_allow_safepoint_count++;
 210     }
 211   }
 212 
 213   ~NoSafepointVerifier() {
 214     if (_activated) {
 215       _thread->_allow_allocation_count--;
 216       _thread->_allow_safepoint_count--;
 217     }
 218   }
 219 #else
 220   NoSafepointVerifier(bool activated = true, bool verifygc = true) : NoGCVerifier(verifygc){}
 221   ~NoSafepointVerifier() {}
 222 #endif
 223 };
 224 
 225 // A PauseNoSafepointVerifier is used to temporarily pause the
 226 // behavior of a NoSafepointVerifier object. If we are not in debug
 227 // mode then there is nothing to do. If the NoSafepointVerifier
 228 // object has an _activated value of false, then there is nothing to
 229 // do for safepoint and allocation checking, but there may still be
 230 // something to do for the underlying NoGCVerifier object.
 231 
 232 class PauseNoSafepointVerifier : public PauseNoGCVerifier {
 233  private:
 234   NoSafepointVerifier * _nsv;
 235 
 236  public:
 237 #ifdef ASSERT
 238   PauseNoSafepointVerifier(NoSafepointVerifier * nsv)
 239     : PauseNoGCVerifier(nsv) {
 240 
 241     _nsv = nsv;
 242     if (_nsv->_activated) {
 243       _nsv->_thread->_allow_allocation_count--;
 244       _nsv->_thread->_allow_safepoint_count--;
 245     }
 246   }
 247 
 248   ~PauseNoSafepointVerifier() {
 249     if (_nsv->_activated) {
 250       _nsv->_thread->_allow_allocation_count++;
 251       _nsv->_thread->_allow_safepoint_count++;
 252     }
 253   }
 254 #else
 255   PauseNoSafepointVerifier(NoSafepointVerifier * nsv)
 256     : PauseNoGCVerifier(nsv) {}
 257   ~PauseNoSafepointVerifier() {}
 258 #endif
 259 };
 260 
 261 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
 262 // over a section of execution by a thread. Currently, it's used only to
 263 // prevent re-entrant calls to GC.
 264 class SkipGCALot : public StackObj {
 265   private:
 266    bool _saved;
 267    Thread* _t;
 268 
 269   public:
 270 #ifdef ASSERT
 271     SkipGCALot(Thread* t) : _t(t) {
 272       _saved = _t->skip_gcalot();
 273       _t->set_skip_gcalot(true);
 274     }
 275 
 276     ~SkipGCALot() {
 277       assert(_t->skip_gcalot(), "Save-restore protocol invariant");
 278       _t->set_skip_gcalot(_saved);
 279     }
 280 #else
 281     SkipGCALot(Thread* t) { }
 282     ~SkipGCALot() { }
 283 #endif
 284 };
 285 
 286 // JRT_LEAF currently can be called from either _thread_in_Java or
 287 // _thread_in_native mode. In _thread_in_native, it is ok
 288 // for another thread to trigger GC. The rest of the JRT_LEAF
 289 // rules apply.
 290 class JRTLeafVerifier : public NoSafepointVerifier {
 291   static bool should_verify_GC();
 292  public:
 293 #ifdef ASSERT
 294   JRTLeafVerifier();
 295   ~JRTLeafVerifier();
 296 #else
 297   JRTLeafVerifier() {}
 298   ~JRTLeafVerifier() {}
 299 #endif
 300 };
 301 
 302 // A NoAllocVerifier object can be placed in methods where one assumes that
 303 // no allocation will occur. The destructor will verify this property
 304 // unless the constructor is called with argument false (not activated).
 305 //
 306 // The check will only be done in debug mode and if activated.
 307 // Note: this only makes sense at safepoints (otherwise, other threads may
 308 // allocate concurrently.)
 309 
 310 class NoAllocVerifier : public StackObj {
 311  private:
 312   bool  _activated;
 313 
 314  public:
 315 #ifdef ASSERT
 316   NoAllocVerifier(bool activated = true) {
 317     _activated = activated;
 318     if (_activated) Thread::current()->_allow_allocation_count++;
 319   }
 320 
 321   ~NoAllocVerifier() {
 322     if (_activated) Thread::current()->_allow_allocation_count--;
 323   }
 324 #else
 325   NoAllocVerifier(bool activated = true) {}
 326   ~NoAllocVerifier() {}
 327 #endif
 328 };
 329 
 330 #endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP