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 33 const uint claimed = UINT_MAX; 34 35 G1FreeIdSet::G1FreeIdSet(uint start, uint size) : 36 _sem(size), // counting semaphore for available ids 37 _next(NULL), // array of "next" indices 38 _start(start), // first id value 39 _size(size), // number of available ids 40 _head_index_mask(0), // mask for extracting index from a _head value. 41 _head(0) // low part: index; high part: update counter 42 { 43 assert(size != 0, "precondition"); 44 assert(start <= (UINT_MAX - size), 45 "start (%u) + size (%u) overflow: ", start, size); 46 // 2^shift must be greater than size. Equal is not permitted, because 47 // size is the "end of list" value, and can be the index part of _head. 48 uint shift = log2_intptr((uintptr_t)size) + 1; 49 assert(shift <= (BitsPerWord / 2), "excessive size %u", size); 50 _head_index_mask = (uintx(1) << shift) - 1; 51 assert(size <= _head_index_mask, "invariant"); 52 _next = NEW_C_HEAP_ARRAY(uint, size, mtGC); 53 for (uint i = 0; i < size; ++i) { 54 _next[i] = i + 1; 55 } 56 } 57 58 G1FreeIdSet::~G1FreeIdSet() { 59 FREE_C_HEAP_ARRAY(uint, _next); 60 } 61 62 uint G1FreeIdSet::head_index(uintx head) const { 63 return head & _head_index_mask; 64 } 65 66 uintx G1FreeIdSet::make_head(uint index, uintx old_head) const { 67 // Include incremented old update counter to avoid ABA problem. 68 return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask); 69 } 70 71 uint 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 90 void 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 }