1 /* 2 * Copyright (c) 2018, 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 #ifndef SHARE_UTILITIES_SINGLEWRITERSYNCHRONIZER_HPP 26 #define SHARE_UTILITIES_SINGLEWRITERSYNCHRONIZER_HPP 27 28 #include "memory/allocation.hpp" 29 #include "runtime/atomic.hpp" 30 #include "runtime/semaphore.hpp" 31 #include "utilities/globalDefinitions.hpp" 32 #include "utilities/macros.hpp" 33 34 // Synchronization primitive inspired by RCU. 35 // 36 // Any number of threads may enter critical sections associated with a 37 // synchronizer object. One (at a time) other thread may wait for the 38 // completion of all critical sections for the synchronizer object 39 // that were extant when the wait was initiated. Usage is that there 40 // is some state that can be accessed either before or after some 41 // change. An accessing thread performs the access within a critical 42 // section. A writer thread performs the state change, and then waits 43 // for critical sections to complete, thereby ensuring there are no 44 // threads in a critical section that might have seen the old state. 45 // 46 // Generally, GlobalCounter should be used instead of this class, as 47 // GlobalCounter has measurably better performance and doesn't have 48 // the single writer at a time restriction. Use this only in 49 // situations where GlobalCounter won't work for some reason. 50 class SingleWriterSynchronizer { 51 volatile uint _enter; 52 volatile uint _exit[2]; 53 volatile uint _waiting_for; 54 Semaphore _wakeup; 55 56 DEBUG_ONLY(volatile uint _writers;) 57 58 NONCOPYABLE(SingleWriterSynchronizer); 59 60 public: 61 SingleWriterSynchronizer(); 62 63 // Enter a critical section for this synchronizer. Entering a 64 // critical section never blocks. While in a critical section, a 65 // thread should avoid blocking, or even take a long time. In 66 // particular, a thread must never safepoint while in a critical 67 // section. 68 // Precondition: The current thread must not already be in a 69 // critical section for this synchronizer. 70 inline uint enter(); 71 72 // Exit a critical section for this synchronizer. 73 // Precondition: enter_value must be the result of the corresponding 74 // enter() for the critical section. 75 inline void exit(uint enter_value); 76 77 // Wait until all threads currently in a critical section for this 78 // synchronizer have exited their critical section. Threads that 79 // enter a critical section after the synchronization has started 80 // are not considered in the wait. 81 // Precondition: No other thread may be synchronizing on this 82 // synchronizer. 83 void synchronize(); 84 85 // RAII class for managing enter/exit pairs. 86 class CriticalSection; 87 }; 88 89 inline uint SingleWriterSynchronizer::enter() { 90 return Atomic::add(&_enter, 2u); 91 } 92 93 inline void SingleWriterSynchronizer::exit(uint enter_value) { 94 uint exit_value = Atomic::add(&_exit[enter_value & 1], 2u); 95 // If this exit completes a synchronize request, wakeup possibly 96 // waiting synchronizer. Read of _waiting_for must follow the _exit 97 // update. 98 if (exit_value == _waiting_for) { 99 _wakeup.signal(); 100 } 101 } 102 103 class SingleWriterSynchronizer::CriticalSection : public StackObj { 104 SingleWriterSynchronizer* _synchronizer; 105 uint _enter_value; 106 107 public: 108 // Enter synchronizer's critical section. 109 explicit CriticalSection(SingleWriterSynchronizer* synchronizer) : 110 _synchronizer(synchronizer), 111 _enter_value(synchronizer->enter()) 112 {} 113 114 // Exit synchronizer's critical section. 115 ~CriticalSection() { 116 _synchronizer->exit(_enter_value); 117 } 118 }; 119 120 #endif // SHARE_UTILITIES_SINGLEWRITERSYNCHRONIZER_HPP