1 /** 2 * @file rwlock.h 3 * 4 * @section LICENSE 5 * 6 * The MIT License 7 * 8 * @copyright Copyright (c) 2021 TileDB, Inc. 9 * 10 * Permission is hereby granted, free of charge, to any person obtaining a copy 11 * of this software and associated documentation files (the "Software"), to deal 12 * in the Software without restriction, including without limitation the rights 13 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell 14 * copies of the Software, and to permit persons to whom the Software is 15 * furnished to do so, subject to the following conditions: 16 * 17 * The above copyright notice and this permission notice shall be included in 18 * all copies or substantial portions of the Software. 19 * 20 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 21 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 22 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 23 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 24 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, 25 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN 26 * THE SOFTWARE. 27 * 28 * @section DESCRIPTION 29 * 30 * This file defines and implements a write-preferring read-write lock. 31 */ 32 33 #ifndef TILEDB_RWLOCK_H 34 #define TILEDB_RWLOCK_H 35 36 #include <condition_variable> 37 #include <mutex> 38 39 #include "tiledb/common/macros.h" 40 41 namespace tiledb { 42 namespace common { 43 44 class RWLock { 45 public: 46 /* ********************************* */ 47 /* CONSTRUCTORS & DESTRUCTORS */ 48 /* ********************************* */ 49 50 /** Default constructor. */ RWLock()51 RWLock() 52 : writer_(false) 53 , waiting_writers_(0) 54 , readers_(0) { 55 } 56 57 /** Default destructor. */ 58 ~RWLock() = default; 59 60 DISABLE_COPY_AND_COPY_ASSIGN(RWLock); 61 DISABLE_MOVE_AND_MOVE_ASSIGN(RWLock); 62 63 /* ********************************* */ 64 /* API */ 65 /* ********************************* */ 66 67 /** Blocks until acquiring a read lock. */ read_lock()68 void read_lock() { 69 std::unique_lock<std::mutex> ul(mutex_); 70 while (waiting_writers_ > 0 || writer_) 71 cv_.wait(ul); 72 ++readers_; 73 } 74 75 /** 76 * Releases a read lock, must be called after 77 * `read_lock`. 78 */ read_unlock()79 void read_unlock() { 80 std::unique_lock<std::mutex> ul(mutex_); 81 if (--readers_ == 0) 82 cv_.notify_all(); 83 } 84 85 /** Blocks until acquiring a write lock. */ write_lock()86 void write_lock() { 87 std::unique_lock<std::mutex> ul(mutex_); 88 ++waiting_writers_; 89 while (writer_ || readers_ > 0) 90 cv_.wait(ul); 91 --waiting_writers_; 92 writer_ = true; 93 } 94 95 /** 96 * Releases a write lock, must be called after 97 * `write_lock`. 98 */ write_unlock()99 void write_unlock() { 100 std::unique_lock<std::mutex> ul(mutex_); 101 writer_ = false; 102 cv_.notify_all(); 103 } 104 105 /** Synchronously downgrades a write lock to a read lock. */ write_downgrade()106 void write_downgrade() { 107 std::unique_lock<std::mutex> ul(mutex_); 108 ++readers_; 109 writer_ = false; 110 cv_.notify_all(); 111 } 112 113 private: 114 /* ********************************* */ 115 /* PRIVATE ATTRIBUTES */ 116 /* ********************************* */ 117 118 /** Protects all member variables. */ 119 std::mutex mutex_; 120 121 /** Conditional variable to signal lock activity. */ 122 std::condition_variable cv_; 123 124 /** True if a write lock is held. */ 125 bool writer_; 126 127 /** The number of writers waiting in `write_lock`. */ 128 uint64_t waiting_writers_; 129 130 /** The number of outstanding read locks. */ 131 uint64_t readers_; 132 }; 133 134 } // namespace common 135 } // namespace tiledb 136 137 #endif // TILEDB_RWLOCK_H 138