1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   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).
  14  *
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  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  24 
  25 #ifndef SHARE_GC_SHARED_GCLOCKER_HPP
  26 #define SHARE_GC_SHARED_GCLOCKER_HPP
  27 
  28 #include "memory/allocation.hpp"
  29 #include "utilities/globalDefinitions.hpp"
  30 #include "utilities/macros.hpp"
  31 
  32 class JavaThread;
  33 
  34 // The direct lock/unlock calls do not force a collection if an unlock
  35 // decrements the count to zero. Avoid calling these if at all possible.
  36 
  37 class GCLocker: public AllStatic {
  38  private:
  39   // The _jni_lock_count keeps track of the number of threads that are
  40   // currently in a critical region.  It's only kept up to date when
  41   // _needs_gc is true.  The current value is computed during
  42   // safepointing and decremented during the slow path of GCLocker
  43   // unlocking.
  44   static volatile jint _jni_lock_count;  // number of jni active instances.
  45   static volatile bool _needs_gc;        // heap is filling, we need a GC
  46                                          // note: bool is typedef'd as jint
  47   static volatile bool _doing_gc;        // unlock_critical() is doing a GC
  48 
  49 #ifdef ASSERT
  50   // This lock count is updated for all operations and is used to
  51   // validate the jni_lock_count that is computed during safepoints.
  52   static volatile jint _debug_jni_lock_count;
  53 #endif
  54 
  55   // At a safepoint, visit all threads and count the number of active
  56   // critical sections.  This is used to ensure that all active
  57   // critical sections are exited before a new one is started.
  58   static void verify_critical_count() NOT_DEBUG_RETURN;
  59 
  60   static void jni_lock(JavaThread* thread);
  61   static void jni_unlock(JavaThread* thread);
  62 
  63   static bool is_active_internal() {
  64     verify_critical_count();
  65     return _jni_lock_count > 0;
  66   }
  67 
  68   static void log_debug_jni(const char* msg);
  69 
  70   static bool is_at_safepoint();
  71 
  72  public:
  73   // Accessors
  74   static bool is_active() {
  75     assert(GCLocker::is_at_safepoint(), "only read at safepoint");
  76     return is_active_internal();
  77   }
  78   static bool needs_gc()       { return _needs_gc;                        }
  79 
  80   // Shorthand
  81   static bool is_active_and_needs_gc() {
  82     // Use is_active_internal since _needs_gc can change from true to
  83     // false outside of a safepoint, triggering the assert in
  84     // is_active.
  85     return needs_gc() && is_active_internal();
  86   }
  87 
  88   // In debug mode track the locking state at all times
  89   static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN;
  90   static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;
  91 
  92   // Set the current lock count
  93   static void set_jni_lock_count(int count) {
  94     _jni_lock_count = count;
  95     verify_critical_count();
  96   }
  97 
  98   // Sets _needs_gc if is_active() is true. Returns is_active().
  99   static bool check_active_before_gc();
 100 
 101   // Stalls the caller (who should not be in a jni critical section)
 102   // until needs_gc() clears. Note however that needs_gc() may be
 103   // set at a subsequent safepoint and/or cleared under the
 104   // JNICritical_lock, so the caller may not safely assert upon
 105   // return from this method that "!needs_gc()" since that is
 106   // not a stable predicate.
 107   static void stall_until_clear();
 108 
 109   // The following two methods are used for JNI critical regions.
 110   // If we find that we failed to perform a GC because the GCLocker
 111   // was active, arrange for one as soon as possible by allowing
 112   // all threads in critical regions to complete, but not allowing
 113   // other critical regions to be entered. The reasons for that are:
 114   // 1) a GC request won't be starved by overlapping JNI critical
 115   //    region activities, which can cause unnecessary OutOfMemory errors.
 116   // 2) even if allocation requests can still be satisfied before GC locker
 117   //    becomes inactive, for example, in tenured generation possibly with
 118   //    heap expansion, those allocations can trigger lots of safepointing
 119   //    attempts (ineffective GC attempts) and require Heap_lock which
 120   //    slow down allocations tremendously.
 121   //
 122   // Note that critical regions can be nested in a single thread, so
 123   // we must allow threads already in critical regions to continue.
 124   //
 125   // JNI critical regions are the only participants in this scheme
 126   // because they are, by spec, well bounded while in a critical region.
 127   //
 128   // Each of the following two method is split into a fast path and a
 129   // slow path. JNICritical_lock is only grabbed in the slow path.
 130   // _needs_gc is initially false and every java thread will go
 131   // through the fast path, which simply increments or decrements the
 132   // current thread's critical count.  When GC happens at a safepoint,
 133   // GCLocker::is_active() is checked. Since there is no safepoint in
 134   // the fast path of lock_critical() and unlock_critical(), there is
 135   // no race condition between the fast path and GC. After _needs_gc
 136   // is set at a safepoint, every thread will go through the slow path
 137   // after the safepoint.  Since after a safepoint, each of the
 138   // following two methods is either entered from the method entry and
 139   // falls into the slow path, or is resumed from the safepoints in
 140   // the method, which only exist in the slow path. So when _needs_gc
 141   // is set, the slow path is always taken, till _needs_gc is cleared.
 142   inline static void lock_critical(JavaThread* thread);
 143   inline static void unlock_critical(JavaThread* thread);
 144 
 145   static address needs_gc_address() { return (address) &_needs_gc; }
 146 };
 147 
 148 #endif // SHARE_GC_SHARED_GCLOCKER_HPP