1 /*
2 * Copyright (c) 2005, 2006, Oracle and/or its affiliates. All rights reserved.
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.
8 *
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 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 // This class describes operations to implement Store-Free Biased
26 // Locking. The high-level properties of the scheme are similar to
27 // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased
28 // locking mechanisms. The principal difference is in the handling of
29 // recursive locking which is how this technique achieves a more
30 // efficient fast path than these other schemes.
31 //
32 // The basic observation is that in HotSpot's current fast locking
33 // scheme, recursive locking (in the fast path) causes no update to
34 // the object header. The recursion is described simply by stack
35 // records containing a specific value (NULL). Only the last unlock by
36 // a given thread causes an update to the object header.
37 //
38 // This observation, coupled with the fact that HotSpot only compiles
39 // methods for which monitor matching is obeyed (and which therefore
40 // can not throw IllegalMonitorStateException), implies that we can
41 // completely eliminate modifications to the object header for
42 // recursive locking in compiled code, and perform similar recursion
43 // checks and throwing of IllegalMonitorStateException in the
44 // interpreter with little or no impact on the performance of the fast
45 // path.
46 //
47 // The basic algorithm is as follows (note, see below for more details
48 // and information). A pattern in the low three bits is reserved in
49 // the object header to indicate whether biasing of a given object's
50 // lock is currently being done or is allowed at all. If the bias
51 // pattern is present, the contents of the rest of the header are
52 // either the JavaThread* of the thread to which the lock is biased,
53 // or NULL, indicating that the lock is "anonymously biased". The
54 // first thread which locks an anonymously biased object biases the
55 // lock toward that thread. If another thread subsequently attempts to
56 // lock the same object, the bias is revoked.
57 //
58 // Because there are no updates to the object header at all during
59 // recursive locking while the lock is biased, the biased lock entry
60 // code is simply a test of the object header's value. If this test
61 // succeeds, the lock has been acquired by the thread. If this test
62 // fails, a bit test is done to see whether the bias bit is still
63 // set. If not, we fall back to HotSpot's original CAS-based locking
64 // scheme. If it is set, we attempt to CAS in a bias toward this
65 // thread. The latter operation is expected to be the rarest operation
66 // performed on these locks. We optimistically expect the biased lock
67 // entry to hit most of the time, and want the CAS-based fallthrough
68 // to occur quickly in the situations where the bias has been revoked.
69 //
70 // Revocation of the lock's bias is fairly straightforward. We want to
71 // restore the object's header and stack-based BasicObjectLocks and
72 // BasicLocks to the state they would have been in had the object been
73 // locked by HotSpot's usual fast locking scheme. To do this, we bring
74 // the system to a safepoint and walk the stack of the thread toward
75 // which the lock is biased. We find all of the lock records on the
76 // stack corresponding to this object, in particular the first /
77 // "highest" record. We fill in the highest lock record with the
78 // object's displaced header (which is a well-known value given that
79 // we don't maintain an identity hash nor age bits for the object
80 // while it's in the biased state) and all other lock records with 0,
81 // the value for recursive locks. When the safepoint is released, the
82 // formerly-biased thread and all other threads revert back to
83 // HotSpot's CAS-based locking.
84 //
85 // This scheme can not handle transfers of biases of single objects
86 // from thread to thread efficiently, but it can handle bulk transfers
87 // of such biases, which is a usage pattern showing up in some
88 // applications and benchmarks. We implement "bulk rebias" and "bulk
89 // revoke" operations using a "bias epoch" on a per-data-type basis.
90 // If too many bias revocations are occurring for a particular data
91 // type, the bias epoch for the data type is incremented at a
92 // safepoint, effectively meaning that all previous biases are
93 // invalid. The fast path locking case checks for an invalid epoch in
94 // the object header and attempts to rebias the object with a CAS if
95 // found, avoiding safepoints or bulk heap sweeps (the latter which
96 // was used in a prior version of this algorithm and did not scale
97 // well). If too many bias revocations persist, biasing is completely
98 // disabled for the data type by resetting the prototype header to the
99 // unbiased markOop. The fast-path locking code checks to see whether
100 // the instance's bias pattern differs from the prototype header's and
101 // causes the bias to be revoked without reaching a safepoint or,
102 // again, a bulk heap sweep.
103
104 // Biased locking counters
105 class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC {
106 private:
107 int _total_entry_count;
108 int _biased_lock_entry_count;
109 int _anonymously_biased_lock_entry_count;
110 int _rebiased_lock_entry_count;
111 int _revoked_lock_entry_count;
112 int _fast_path_entry_count;
113 int _slow_path_entry_count;
114
115 public:
116 BiasedLockingCounters() :
117 _total_entry_count(0),
118 _biased_lock_entry_count(0),
119 _anonymously_biased_lock_entry_count(0),
120 _rebiased_lock_entry_count(0),
121 _revoked_lock_entry_count(0),
122 _fast_path_entry_count(0),
123 _slow_path_entry_count(0) {}
124
125 int slow_path_entry_count(); // Compute this field if necessary
126
127 int* total_entry_count_addr() { return &_total_entry_count; }
128 int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; }
129 int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }
130 int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; }
131 int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; }
132 int* fast_path_entry_count_addr() { return &_fast_path_entry_count; }
133 int* slow_path_entry_count_addr() { return &_slow_path_entry_count; }
134
135 bool nonzero() { return _total_entry_count > 0; }
136
137 void print_on(outputStream* st);
138 void print() { print_on(tty); }
139 };
140
141
142 class BiasedLocking : AllStatic {
143 private:
144 static BiasedLockingCounters _counters;
145
146 public:
147 static int* total_entry_count_addr();
148 static int* biased_lock_entry_count_addr();
149 static int* anonymously_biased_lock_entry_count_addr();
150 static int* rebiased_lock_entry_count_addr();
151 static int* revoked_lock_entry_count_addr();
152 static int* fast_path_entry_count_addr();
153 static int* slow_path_entry_count_addr();
154
155 enum Condition {
156 NOT_BIASED = 1,
157 BIAS_REVOKED = 2,
158 BIAS_REVOKED_AND_REBIASED = 3
159 };
160
161 // This initialization routine should only be called once and
162 // schedules a PeriodicTask to turn on biased locking a few seconds
163 // into the VM run to avoid startup time regressions
164 static void init();
165
166 // This provides a global switch for leaving biased locking disabled
167 // for the first part of a run and enabling it later
168 static bool enabled();
169
170 // This should be called by JavaThreads to revoke the bias of an object
171 static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);
172
173 // These do not allow rebiasing; they are used by deoptimization to
174 // ensure that monitors on the stack can be migrated
175 static void revoke(GrowableArray<Handle>* objs);
176 static void revoke_at_safepoint(Handle obj);
177 static void revoke_at_safepoint(GrowableArray<Handle>* objs);
178
179 static void print_counters() { _counters.print(); }
180 static BiasedLockingCounters* counters() { return &_counters; }
181
182 // These routines are GC-related and should not be called by end
183 // users. GCs which do not do preservation of mark words do not need
184 // to call these routines.
185 static void preserve_marks();
186 static void restore_marks();
187 };