1/*
2 * Copyright (c) 2019, 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#include "precompiled.hpp"
26#include "gc/g1/g1FreeIdSet.hpp"
27#include "memory/allocation.inline.hpp"
28#include "runtime/atomic.hpp"
29#include "utilities/debug.hpp"
30#include "utilities/globalDefinitions.hpp"
31#include "utilities/macros.hpp"
32
33G1FreeIdSet::G1FreeIdSet(uint start, uint size) :
34 _sem(size), // counting semaphore for available ids
35 _next(NULL), // array of "next" indices
36 _start(start), // first id value
37 _size(size), // number of available ids
38 _head_index_mask(0), // mask for extracting index from a _head value.
39 _head(0) // low part: index; high part: update counter
40{
41 assert(size != 0, "precondition");
42 assert(start <= (UINT_MAX - size),
43 "start (%u) + size (%u) overflow: ", start, size);
44 // 2^shift must be greater than size. Equal is not permitted, because
45 // size is the "end of list" value, and can be the index part of _head.
46 uint shift = log2_intptr((uintptr_t)size) + 1;
47 assert(shift <= (BitsPerWord / 2), "excessive size %u", size);
48 _head_index_mask = (uintx(1) << shift) - 1;
49 assert(size <= _head_index_mask, "invariant");
50 _next = NEW_C_HEAP_ARRAY(uint, size, mtGC);
51 for (uint i = 0; i < size; ++i) {
52 _next[i] = i + 1;
53 }
54}
55
56G1FreeIdSet::~G1FreeIdSet() {
57 FREE_C_HEAP_ARRAY(uint, _next);
58}
59
60uint G1FreeIdSet::head_index(uintx head) const {
61 return head & _head_index_mask;
62}
63
64uintx G1FreeIdSet::make_head(uint index, uintx old_head) const {
65 // Include incremented old update counter to avoid ABA problem.
66 return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask);
67}
68
69const uint Claimed = UINT_MAX;
70
71uint G1FreeIdSet::claim_par_id() {
72 _sem.wait();
73 // Semaphore gate permits passage by no more than the number of
74 // available ids, so there must be one that we can claim. But there
75 // may be multiple threads trying to claim ids at the same time.
76 uintx old_head = Atomic::load(&_head);
77 uint index;
78 while (true) {
79 index = head_index(old_head);
80 assert(index < _size, "invariant");
81 uintx new_head = make_head(_next[index], old_head);
82 new_head = Atomic::cmpxchg(new_head, &_head, old_head);
83 if (new_head == old_head) break;
84 old_head = new_head;
85 }
86 DEBUG_ONLY(_next[index] = Claimed;)
87 return _start + index;
88}
89
90void G1FreeIdSet::release_par_id(uint id) {
91 uint index = id - _start;
92 assert(index < _size, "invalid id %u", id);
93 assert(_next[index] == Claimed, "precondition");
94 uintx old_head = Atomic::load(&_head);
95 while (true) {
96 _next[index] = head_index(old_head);
97 uintx new_head = make_head(index, old_head);
98 new_head = Atomic::cmpxchg(new_head, &_head, old_head);
99 if (new_head == old_head) break;
100 old_head = new_head;
101 }
102 // Now that id has been released, permit another thread through the gate.
103 _sem.signal();
104}
105