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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
32class 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
37class 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
149