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