1 /*
2 * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
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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7 * published by the Free Software Foundation.
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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).
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23 */
24
25 #ifndef SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
26 #define SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
27
28 #include "gc/shared/oopStorage.hpp"
29 #include "utilities/macros.hpp"
30
31 //////////////////////////////////////////////////////////////////////////////
32 // Support for parallel and optionally concurrent state iteration.
33 //
34 // Parallel iteration is for the exclusive use of the GC. Other iteration
35 // clients must use serial iteration.
36 //
37 // Concurrent Iteration
38 //
39 // Iteration involves the _active_list, which contains all of the blocks owned
40 // by a storage object. This is a doubly-linked list, linked through
41 // dedicated fields in the blocks.
42 //
43 // At most one concurrent ParState can exist at a time for a given storage
44 // object.
45 //
46 // A concurrent ParState sets the associated storage's
47 // _concurrent_iteration_active flag true when the state is constructed, and
48 // sets it false when the state is destroyed. These assignments are made with
49 // _active_mutex locked. Meanwhile, empty block deletion is not done while
50 // _concurrent_iteration_active is true. The flag check and the dependent
51 // removal of a block from the _active_list is performed with _active_mutex
52 // locked. This prevents concurrent iteration and empty block deletion from
53 // interfering with with each other.
54 //
55 // Both allocate() and delete_empty_blocks_concurrent() lock the
56 // _allocate_mutex while performing their respective list manipulations,
57 // preventing them from interfering with each other.
58 //
59 // When allocate() creates a new block, it is added to the front of the
60 // _active_list. Then _active_head is set to the new block. When concurrent
61 // iteration is started (by a parallel worker thread calling the state's
62 // iterate() function), the current _active_head is used as the initial block
63 // for the iteration, with iteration proceeding down the list headed by that
64 // block.
65 //
66 // As a result, the list over which concurrent iteration operates is stable.
67 // However, once the iteration is started, later allocations may add blocks to
68 // the front of the list that won't be examined by the iteration. And while
69 // the list is stable, concurrent allocate() and release() operations may
70 // change the set of allocated entries in a block at any time during the
71 // iteration.
72 //
73 // As a result, a concurrent iteration handler must accept that some
74 // allocations and releases that occur after the iteration started will not be
75 // seen by the iteration. Further, some may overlap examination by the
76 // iteration. To help with this, allocate() and release() have an invariant
77 // that an entry's value must be NULL when it is not in use.
78 //
79 // An in-progress delete_empty_blocks_concurrent() operation can contend with
80 // the start of a concurrent iteration over the _active_mutex. Since both are
81 // under GC control, that potential contention can be eliminated by never
82 // scheduling both operations to run at the same time.
83 //
84 // ParState<concurrent, is_const>
85 // concurrent must be true if iteration is concurrent with the
86 // mutator, false if iteration is at a safepoint.
87 //
88 // is_const must be true if the iteration is over a constant storage
89 // object, false if the iteration may modify the storage object.
90 //
91 // ParState([const] OopStorage* storage)
92 // Construct an object for managing an iteration over storage. For a
93 // concurrent ParState, empty block deletion for the associated storage
94 // is inhibited for the life of the ParState. There can be no more
95 // than one live concurrent ParState at a time for a given storage object.
96 //
97 // template<typename F> void iterate(F f)
98 // Repeatedly claims a block from the associated storage that has
99 // not been processed by this iteration (possibly by other threads),
100 // and applies f to each entry in the claimed block. Assume p is of
101 // type const oop* or oop*, according to is_const. Then f(p) must be
102 // a valid expression whose value is ignored. Concurrent uses must
103 // be prepared for an entry's value to change at any time, due to
104 // mutator activity.
105 //
106 // template<typename Closure> void oops_do(Closure* cl)
107 // Wrapper around iterate, providing an adaptation layer allowing
108 // the use of OopClosures and similar objects for iteration. Assume
109 // p is of type const oop* or oop*, according to is_const. Then
110 // cl->do_oop(p) must be a valid expression whose value is ignored.
111 // Concurrent uses must be prepared for the entry's value to change
112 // at any time, due to mutator activity.
113 //
114 // Optional operations, provided only if !concurrent && !is_const.
115 // These are not provided when is_const, because the storage object
116 // may be modified by the iteration infrastructure, even if the
117 // provided closure doesn't modify the storage object. These are not
118 // provided when concurrent because any pre-filtering behavior by the
119 // iteration infrastructure is inappropriate for concurrent iteration;
120 // modifications of the storage by the mutator could result in the
121 // pre-filtering being applied (successfully or not) to objects that
122 // are unrelated to what the closure finds in the entry.
123 //
124 // template<typename Closure> void weak_oops_do(Closure* cl)
125 // template<typename IsAliveClosure, typename Closure>
126 // void weak_oops_do(IsAliveClosure* is_alive, Closure* cl)
127 // Wrappers around iterate, providing an adaptation layer allowing
128 // the use of is-alive closures and OopClosures for iteration.
129 // Assume p is of type oop*. Then
130 //
131 // - cl->do_oop(p) must be a valid expression whose value is ignored.
132 //
133 // - is_alive->do_object_b(*p) must be a valid expression whose value
134 // is convertible to bool.
135 //
136 // If *p == NULL then neither is_alive nor cl will be invoked for p.
137 // If is_alive->do_object_b(*p) is false, then cl will not be
138 // invoked on p.
139
140 class OopStorage::BasicParState {
141 OopStorage* _storage;
142 void* volatile _next_block;
143 bool _concurrent;
144
145 // Noncopyable.
146 BasicParState(const BasicParState&);
147 BasicParState& operator=(const BasicParState&);
148
149 void update_iteration_state(bool value);
150 void ensure_iteration_started();
151 Block* claim_next_block();
152
153 // Wrapper for iteration handler; ignore handler result and return true.
154 template<typename F> class AlwaysTrueFn;
155
156 public:
157 BasicParState(OopStorage* storage, bool concurrent);
158 ~BasicParState();
159
160 template<bool is_const, typename F> void iterate(F f);
161 };
162
163 template<bool concurrent, bool is_const>
164 class OopStorage::ParState {
165 BasicParState _basic_state;
166
167 public:
168 ParState(const OopStorage* storage) :
169 // For simplicity, always recorded as non-const.
170 _basic_state(const_cast<OopStorage*>(storage), concurrent)
171 {}
172
173 template<typename F> void iterate(F f);
174 template<typename Closure> void oops_do(Closure* cl);
175 };
176
177 template<>
178 class OopStorage::ParState<false, false> {
179 BasicParState _basic_state;
180
181 public:
182 ParState(OopStorage* storage) :
183 _basic_state(storage, false)
184 {}
185
186 template<typename F> void iterate(F f);
187 template<typename Closure> void oops_do(Closure* cl);
188 template<typename Closure> void weak_oops_do(Closure* cl);
189 template<typename IsAliveClosure, typename Closure>
190 void weak_oops_do(IsAliveClosure* is_alive, Closure* cl);
191 };
192
193 #endif // SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP