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24
25#ifndef SHARE_RUNTIME_MUTEX_HPP
26#define SHARE_RUNTIME_MUTEX_HPP
27
28#include "memory/allocation.hpp"
29#include "runtime/os.hpp"
30
31// A Mutex/Monitor is a simple wrapper around a native lock plus condition
32// variable that supports lock ownership tracking, lock ranking for deadlock
33// detection and coordinates with the safepoint protocol.
34
35// The default length of monitor name was originally chosen to be 64 to avoid
36// false sharing. Now, PaddedMonitor is available for this purpose.
37// TODO: Check if _name[MONITOR_NAME_LEN] should better get replaced by const char*.
38static const int MONITOR_NAME_LEN = 64;
39
40class Monitor : public CHeapObj<mtSynchronizer> {
41
42 public:
43 // A special lock: Is a lock where you are guaranteed not to block while you are
44 // holding it, i.e., no vm operation can happen, taking other (blocking) locks, etc.
45 // The rank 'access' is similar to 'special' and has the same restrictions on usage.
46 // It is reserved for locks that may be required in order to perform memory accesses
47 // that require special barriers, e.g. SATB GC barriers, that in turn uses locks.
48 // The rank 'tty' is also similar to 'special' and has the same restrictions.
49 // It is reserved for the tty_lock.
50 // Since memory accesses should be able to be performed pretty much anywhere
51 // in the code, that requires locks required for performing accesses being
52 // inherently a bit more special than even locks of the 'special' rank.
53 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
54 // (except for "event" and "access") for the deadlock detection to work correctly.
55 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
56 // which being external to the VM are not subject to deadlock detection.
57 // While at a safepoint no mutexes of rank safepoint are held by any thread.
58 // The rank named "leaf" is probably historical (and should
59 // be changed) -- mutexes of this rank aren't really leaf mutexes
60 // at all.
61 enum lock_types {
62 event,
63 access = event + 1,
64 tty = access + 2,
65 special = tty + 1,
66 suspend_resume = special + 1,
67 vmweak = suspend_resume + 2,
68 leaf = vmweak + 2,
69 safepoint = leaf + 10,
70 barrier = safepoint + 1,
71 nonleaf = barrier + 1,
72 max_nonleaf = nonleaf + 900,
73 native = max_nonleaf + 1
74 };
75
76 protected: // Monitor-Mutex metadata
77 Thread * volatile _owner; // The owner of the lock
78 os::PlatformMonitor _lock; // Native monitor implementation
79 char _name[MONITOR_NAME_LEN]; // Name of mutex/monitor
80
81 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
82#ifndef PRODUCT
83 bool _allow_vm_block;
84 DEBUG_ONLY(int _rank;) // rank (to avoid/detect potential deadlocks)
85 DEBUG_ONLY(Monitor * _next;) // Used by a Thread to link up owned locks
86 DEBUG_ONLY(Thread* _last_owner;) // the last thread to own the lock
87 DEBUG_ONLY(static bool contains(Monitor * locks, Monitor * lock);)
88 DEBUG_ONLY(static Monitor * get_least_ranked_lock(Monitor * locks);)
89 DEBUG_ONLY(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
90#endif
91
92 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
93 void check_prelock_state (Thread* thread, bool safepoint_check) PRODUCT_RETURN;
94 void check_block_state (Thread* thread) PRODUCT_RETURN;
95 void check_safepoint_state (Thread* thread, bool safepoint_check) NOT_DEBUG_RETURN;
96 void assert_owner (Thread* expected) NOT_DEBUG_RETURN;
97 void assert_wait_lock_state (Thread* self) NOT_DEBUG_RETURN;
98
99 public:
100 enum {
101 _allow_vm_block_flag = true,
102 _as_suspend_equivalent_flag = true
103 };
104
105 // Locks can be acquired with or without a safepoint check. NonJavaThreads do not follow
106 // the safepoint protocol when acquiring locks.
107
108 // Each lock can be acquired by only JavaThreads, only NonJavaThreads, or shared between
109 // Java and NonJavaThreads. When the lock is initialized with _safepoint_check_always,
110 // that means that whenever the lock is acquired by a JavaThread, it will verify that
111 // it is done with a safepoint check. In corollary, when the lock is initialized with
112 // _safepoint_check_never, that means that whenever the lock is acquired by a JavaThread
113 // it will verify that it is done without a safepoint check.
114
115
116 // There are a couple of existing locks that will sometimes have a safepoint check and
117 // sometimes not when acquired by a JavaThread, but these locks are set up carefully
118 // to avoid deadlocks. TODO: Fix these locks and remove _safepoint_check_sometimes.
119
120 // TODO: Locks that are shared between JavaThreads and NonJavaThreads
121 // should never encounter a safepoint check while they are held, or else a
122 // deadlock can occur. We should check this by noting which
123 // locks are shared, and walk held locks during safepoint checking.
124
125 enum SafepointCheckFlag {
126 _safepoint_check_flag,
127 _no_safepoint_check_flag
128 };
129
130 enum SafepointCheckRequired {
131 _safepoint_check_never, // Monitors with this value will cause errors
132 // when acquired by a JavaThread with a safepoint check.
133 _safepoint_check_sometimes, // A couple of special locks are acquired by JavaThreads sometimes
134 // with and sometimes without safepoint checks. These
135 // locks will not produce errors when locked.
136 _safepoint_check_always // Monitors with this value will cause errors
137 // when acquired by a JavaThread without a safepoint check.
138 };
139
140 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)
141
142 protected:
143 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
144 Monitor() ;
145
146 public:
147 Monitor(int rank, const char *name, bool allow_vm_block = false,
148 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
149 ~Monitor();
150
151 // Wait until monitor is notified (or times out).
152 // Defaults are to make safepoint checks, wait time is forever (i.e.,
153 // zero), and not a suspend-equivalent condition. Returns true if wait
154 // times out; otherwise returns false.
155 bool wait(long timeout = 0,
156 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
157 bool wait_without_safepoint_check(long timeout = 0);
158 void notify();
159 void notify_all();
160
161
162 void lock(); // prints out warning if VM thread blocks
163 void lock(Thread *thread); // overloaded with current thread
164 void unlock();
165 bool is_locked() const { return _owner != NULL; }
166
167 bool try_lock(); // Like lock(), but unblocking. It returns false instead
168
169 void release_for_safepoint();
170
171 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
172 // that is guaranteed not to block while running inside the VM.
173 void lock_without_safepoint_check();
174 void lock_without_safepoint_check(Thread* self);
175
176 // Current owner - not not MT-safe. Can only be used to guarantee that
177 // the current running thread owns the lock
178 Thread* owner() const { return _owner; }
179 bool owned_by_self() const;
180
181 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
182 // non-Java thread. (We should really have a RawMonitor abstraction)
183 void jvm_raw_lock();
184 void jvm_raw_unlock();
185 const char *name() const { return _name; }
186
187 void print_on_error(outputStream* st) const;
188
189 #ifndef PRODUCT
190 void print_on(outputStream* st) const;
191 void print() const { print_on(::tty); }
192 DEBUG_ONLY(int rank() const { return _rank; })
193 bool allow_vm_block() { return _allow_vm_block; }
194
195 DEBUG_ONLY(Monitor *next() const { return _next; })
196 DEBUG_ONLY(void set_next(Monitor *next) { _next = next; })
197 #endif
198
199 void set_owner(Thread* owner) {
200 #ifndef PRODUCT
201 set_owner_implementation(owner);
202 DEBUG_ONLY(void verify_Monitor(Thread* thr);)
203 #else
204 _owner = owner;
205 #endif
206 }
207
208};
209
210class PaddedMonitor : public Monitor {
211 enum {
212 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor),
213 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
214 };
215 char _padding[PADDING_LEN];
216 public:
217 PaddedMonitor(int rank, const char *name, bool allow_vm_block = false,
218 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
219 Monitor(rank, name, allow_vm_block, safepoint_check_required) {};
220};
221
222// Normally we'd expect Monitor to extend Mutex in the sense that a monitor
223// constructed from pthreads primitives might extend a mutex by adding
224// a condvar and some extra metadata. In fact this was the case until J2SE7.
225//
226// Currently, however, the base object is a monitor. Monitor contains all the
227// logic for wait(), notify(), etc. Mutex extends monitor and restricts the
228// visibility of wait(), notify(), and notify_all().
229//
230// Another viable alternative would have been to have Monitor extend Mutex and
231// implement all the normal mutex and wait()-notify() logic in Mutex base class.
232// The wait()-notify() facility would be exposed via special protected member functions
233// (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
234// as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
235// _Wait().
236//
237// An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
238// After all, monitors are sufficient for Java-level synchronization. At one point in time
239// there may have been some benefit to having distinct mutexes and monitors, but that time
240// has passed.
241//
242
243class Mutex : public Monitor { // degenerate Monitor
244 public:
245 Mutex(int rank, const char *name, bool allow_vm_block = false,
246 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
247 // default destructor
248 private:
249 void notify();
250 void notify_all();
251 bool wait(long timeout, bool as_suspend_equivalent);
252 bool wait_without_safepoint_check(long timeout);
253};
254
255class PaddedMutex : public Mutex {
256 enum {
257 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex),
258 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
259 };
260 char _padding[PADDING_LEN];
261public:
262 PaddedMutex(int rank, const char *name, bool allow_vm_block = false,
263 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
264 Mutex(rank, name, allow_vm_block, safepoint_check_required) {};
265};
266
267#endif // SHARE_RUNTIME_MUTEX_HPP
268