| /* |
| * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| * |
| */ |
| |
| #include "precompiled.hpp" |
| #include "gc/g1/g1FreeIdSet.hpp" |
| #include "memory/allocation.inline.hpp" |
| #include "runtime/atomic.hpp" |
| #include "utilities/debug.hpp" |
| #include "utilities/globalDefinitions.hpp" |
| #include "utilities/macros.hpp" |
| |
| G1FreeIdSet::G1FreeIdSet(uint start, uint size) : |
| _sem(size), // counting semaphore for available ids |
| _next(NULL), // array of "next" indices |
| _start(start), // first id value |
| _size(size), // number of available ids |
| _head_index_mask(0), // mask for extracting index from a _head value. |
| _head(0) // low part: index; high part: update counter |
| { |
| assert(size != 0, "precondition"); |
| assert(start <= (UINT_MAX - size), |
| "start (%u) + size (%u) overflow: ", start, size); |
| // 2^shift must be greater than size. Equal is not permitted, because |
| // size is the "end of list" value, and can be the index part of _head. |
| uint shift = log2_intptr((uintptr_t)size) + 1; |
| assert(shift <= (BitsPerWord / 2), "excessive size %u", size); |
| _head_index_mask = (uintx(1) << shift) - 1; |
| assert(size <= _head_index_mask, "invariant"); |
| _next = NEW_C_HEAP_ARRAY(uint, size, mtGC); |
| for (uint i = 0; i < size; ++i) { |
| _next[i] = i + 1; |
| } |
| } |
| |
| G1FreeIdSet::~G1FreeIdSet() { |
| FREE_C_HEAP_ARRAY(uint, _next); |
| } |
| |
| uint G1FreeIdSet::head_index(uintx head) const { |
| return head & _head_index_mask; |
| } |
| |
| uintx G1FreeIdSet::make_head(uint index, uintx old_head) const { |
| // Include incremented old update counter to avoid ABA problem. |
| return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask); |
| } |
| |
| const uint Claimed = UINT_MAX; |
| |
| uint G1FreeIdSet::claim_par_id() { |
| _sem.wait(); |
| // Semaphore gate permits passage by no more than the number of |
| // available ids, so there must be one that we can claim. But there |
| // may be multiple threads trying to claim ids at the same time. |
| uintx old_head = Atomic::load(&_head); |
| uint index; |
| while (true) { |
| index = head_index(old_head); |
| assert(index < _size, "invariant"); |
| uintx new_head = make_head(_next[index], old_head); |
| new_head = Atomic::cmpxchg(new_head, &_head, old_head); |
| if (new_head == old_head) break; |
| old_head = new_head; |
| } |
| DEBUG_ONLY(_next[index] = Claimed;) |
| return _start + index; |
| } |
| |
| void G1FreeIdSet::release_par_id(uint id) { |
| uint index = id - _start; |
| assert(index < _size, "invalid id %u", id); |
| assert(_next[index] == Claimed, "precondition"); |
| uintx old_head = Atomic::load(&_head); |
| while (true) { |
| _next[index] = head_index(old_head); |
| uintx new_head = make_head(index, old_head); |
| new_head = Atomic::cmpxchg(new_head, &_head, old_head); |
| if (new_head == old_head) break; |
| old_head = new_head; |
| } |
| // Now that id has been released, permit another thread through the gate. |
| _sem.signal(); |
| } |