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.
4 *
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 #if INCLUDE_ALL_GCS
32
33 //////////////////////////////////////////////////////////////////////////////
34 // Support for parallel and optionally concurrent state iteration.
35 //
36 // Parallel iteration is for the exclusive use of the GC. Other iteration
37 // clients must use serial iteration.
38 //
39 // Concurrent Iteration
40 //
41 // Iteration involves the _active_list, which contains all of the blocks owned
42 // by a storage object. This is a doubly-linked list, linked through
43 // dedicated fields in the blocks.
44 //
45 // At most one concurrent ParState can exist at a time for a given storage
46 // object.
47 //
48 // A concurrent ParState sets the associated storage's
49 // _concurrent_iteration_active flag true when the state is constructed, and
50 // sets it false when the state is destroyed. These assignments are made with
51 // _active_mutex locked. Meanwhile, empty block deletion is not done while
52 // _concurrent_iteration_active is true. The flag check and the dependent
53 // removal of a block from the _active_list is performed with _active_mutex
54 // locked. This prevents concurrent iteration and empty block deletion from
55 // interfering with with each other.
56 //
57 // Both allocate() and delete_empty_blocks_concurrent() lock the
58 // _allocate_mutex while performing their respective list manipulations,
59 // preventing them from interfering with each other.
60 //
61 // When allocate() creates a new block, it is added to the front of the
62 // _active_list. Then _active_head is set to the new block. When concurrent
63 // iteration is started (by a parallel worker thread calling the state's
64 // iterate() function), the current _active_head is used as the initial block
65 // for the iteration, with iteration proceeding down the list headed by that
66 // block.
67 //
68 // As a result, the list over which concurrent iteration operates is stable.
69 // However, once the iteration is started, later allocations may add blocks to
70 // the front of the list that won't be examined by the iteration. And while
71 // the list is stable, concurrent allocate() and release() operations may
72 // change the set of allocated entries in a block at any time during the
73 // iteration.
74 //
75 // As a result, a concurrent iteration handler must accept that some
76 // allocations and releases that occur after the iteration started will not be
77 // seen by the iteration. Further, some may overlap examination by the
78 // iteration. To help with this, allocate() and release() have an invariant
79 // that an entry's value must be NULL when it is not in use.
80 //
81 // An in-progress delete_empty_blocks_concurrent() operation can contend with
82 // the start of a concurrent iteration over the _active_mutex. Since both are
83 // under GC control, that potential contention can be eliminated by never
84 // scheduling both operations to run at the same time.
85 //
86 // ParState<concurrent, is_const>
87 // concurrent must be true if iteration is concurrent with the
88 // mutator, false if iteration is at a safepoint.
89 //
90 // is_const must be true if the iteration is over a constant storage
91 // object, false if the iteration may modify the storage object.
92 //
93 // ParState([const] OopStorage* storage)
94 // Construct an object for managing an iteration over storage. For a
95 // concurrent ParState, empty block deletion for the associated storage
96 // is inhibited for the life of the ParState. There can be no more
97 // than one live concurrent ParState at a time for a given storage object.
98 //
99 // template<typename F> void iterate(F f)
100 // Repeatedly claims a block from the associated storage that has
101 // not been processed by this iteration (possibly by other threads),
102 // and applies f to each entry in the claimed block. Assume p is of
103 // type const oop* or oop*, according to is_const. Then f(p) must be
104 // a valid expression whose value is ignored. Concurrent uses must
105 // be prepared for an entry's value to change at any time, due to
106 // mutator activity.
107 //
108 // template<typename Closure> void oops_do(Closure* cl)
109 // Wrapper around iterate, providing an adaptation layer allowing
110 // the use of OopClosures and similar objects for iteration. Assume
111 // p is of type const oop* or oop*, according to is_const. Then
112 // cl->do_oop(p) must be a valid expression whose value is ignored.
113 // Concurrent uses must be prepared for the entry's value to change
114 // at any time, due to mutator activity.
115 //
116 // Optional operations, provided only if !concurrent && !is_const.
117 // These are not provided when is_const, because the storage object
118 // may be modified by the iteration infrastructure, even if the
119 // provided closure doesn't modify the storage object. These are not
120 // provided when concurrent because any pre-filtering behavior by the
121 // iteration infrastructure is inappropriate for concurrent iteration;
122 // modifications of the storage by the mutator could result in the
123 // pre-filtering being applied (successfully or not) to objects that
124 // are unrelated to what the closure finds in the entry.
125 //
126 // template<typename Closure> void weak_oops_do(Closure* cl)
127 // template<typename IsAliveClosure, typename Closure>
128 // void weak_oops_do(IsAliveClosure* is_alive, Closure* cl)
129 // Wrappers around iterate, providing an adaptation layer allowing
130 // the use of is-alive closures and OopClosures for iteration.
131 // Assume p is of type oop*. Then
132 //
133 // - cl->do_oop(p) must be a valid expression whose value is ignored.
134 //
135 // - is_alive->do_object_b(*p) must be a valid expression whose value
136 // is convertible to bool.
137 //
138 // If *p == NULL then neither is_alive nor cl will be invoked for p.
139 // If is_alive->do_object_b(*p) is false, then cl will not be
140 // invoked on p.
141
142 class OopStorage::BasicParState {
143 OopStorage* _storage;
144 void* volatile _next_block;
145 bool _concurrent;
146
147 // Noncopyable.
148 BasicParState(const BasicParState&);
149 BasicParState& operator=(const BasicParState&);
150
151 void update_iteration_state(bool value);
152 void ensure_iteration_started();
153 Block* claim_next_block();
154
155 // Wrapper for iteration handler; ignore handler result and return true.
156 template<typename F> class AlwaysTrueFn;
157
158 public:
159 BasicParState(OopStorage* storage, bool concurrent);
160 ~BasicParState();
161
162 template<bool is_const, typename F> void iterate(F f);
163 };
164
165 template<bool concurrent, bool is_const>
166 class OopStorage::ParState {
167 BasicParState _basic_state;
168
169 public:
170 ParState(const OopStorage* storage) :
171 // For simplicity, always recorded as non-const.
172 _basic_state(const_cast<OopStorage*>(storage), concurrent)
173 {}
174
175 template<typename F> void iterate(F f);
176 template<typename Closure> void oops_do(Closure* cl);
177 };
178
179 template<>
180 class OopStorage::ParState<false, false> {
181 BasicParState _basic_state;
182
183 public:
184 ParState(OopStorage* storage) :
185 _basic_state(storage, false)
186 {}
187
188 template<typename F> void iterate(F f);
189 template<typename Closure> void oops_do(Closure* cl);
190 template<typename Closure> void weak_oops_do(Closure* cl);
191 template<typename IsAliveClosure, typename Closure>
192 void weak_oops_do(IsAliveClosure* is_alive, Closure* cl);
193 };
194
195 #endif // INCLUDE_ALL_GCS
196
197 #endif // SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP