The following class implements a reader-writer lock to use in the second readers-writers problem with python threads. In this problem, many readers can simultaneously access a share, and a writer has an exclusive access to this share. Additionally, the following constraints should be met: 1) no reader should be kept waiting if the share is currently opened for reading unless a writer is also waiting for the share, 2) no writer should be kept waiting for the share longer than absolutely necessary.
Python, 247 lines
Download
Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 | import threading
__author__ = "Mateusz Kobos"
class RWLock:
"""Synchronization object used in a solution of so-called second
readers-writers problem. In this problem, many readers can simultaneously
access a share, and a writer has an exclusive access to this share.
Additionally, the following constraints should be met:
1) no reader should be kept waiting if the share is currently opened for
reading unless a writer is also waiting for the share,
2) no writer should be kept waiting for the share longer than absolutely
necessary.
The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7]
with a modification -- adding an additional lock (C{self.__readers_queue})
-- in accordance with [2].
Sources:
[1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008
[2] P.J. Courtois, F. Heymans, D.L. Parnas:
"Concurrent Control with 'Readers' and 'Writers'",
Communications of the ACM, 1971 (via [3])
[3] http://en.wikipedia.org/wiki/Readers-writers_problem
"""
def __init__(self):
self.__read_switch = _LightSwitch()
self.__write_switch = _LightSwitch()
self.__no_readers = threading.Lock()
self.__no_writers = threading.Lock()
self.__readers_queue = threading.Lock()
"""A lock giving an even higher priority to the writer in certain
cases (see [2] for a discussion)"""
def reader_acquire(self):
self.__readers_queue.acquire()
self.__no_readers.acquire()
self.__read_switch.acquire(self.__no_writers)
self.__no_readers.release()
self.__readers_queue.release()
def reader_release(self):
self.__read_switch.release(self.__no_writers)
def writer_acquire(self):
self.__write_switch.acquire(self.__no_readers)
self.__no_writers.acquire()
def writer_release(self):
self.__no_writers.release()
self.__write_switch.release(self.__no_readers)
class _LightSwitch:
"""An auxiliary "light switch"-like object. The first thread turns on the
"switch", the last one turns it off (see [1, sec. 4.2.2] for details)."""
def __init__(self):
self.__counter = 0
self.__mutex = threading.Lock()
def acquire(self, lock):
self.__mutex.acquire()
self.__counter += 1
if self.__counter == 1:
lock.acquire()
self.__mutex.release()
def release(self, lock):
self.__mutex.acquire()
self.__counter -= 1
if self.__counter == 0:
lock.release()
self.__mutex.release()
##
## Unit testing code
## =================
##
import unittest
import threading
import time
import copy
class Writer(threading.Thread):
def __init__(self, buffer_, rw_lock, init_sleep_time, sleep_time, to_write):
"""
@param buffer_: common buffer_ shared by the readers and writers
@type buffer_: list
@type rw_lock: L{RWLock}
@param init_sleep_time: sleep time before doing any action
@type init_sleep_time: C{float}
@param sleep_time: sleep time while in critical section
@type sleep_time: C{float}
@param to_write: data that will be appended to the buffer
"""
threading.Thread.__init__(self)
self.__buffer = buffer_
self.__rw_lock = rw_lock
self.__init_sleep_time = init_sleep_time
self.__sleep_time = sleep_time
self.__to_write = to_write
self.entry_time = None
"""Time of entry to the critical section"""
self.exit_time = None
"""Time of exit from the critical section"""
def run(self):
time.sleep(self.__init_sleep_time)
self.__rw_lock.writer_acquire()
self.entry_time = time.time()
time.sleep(self.__sleep_time)
self.__buffer.append(self.__to_write)
self.exit_time = time.time()
self.__rw_lock.writer_release()
class Reader(threading.Thread):
def __init__(self, buffer_, rw_lock, init_sleep_time, sleep_time):
"""
@param buffer_: common buffer shared by the readers and writers
@type buffer_: list
@type rw_lock: L{RWLock}
@param init_sleep_time: sleep time before doing any action
@type init_sleep_time: C{float}
@param sleep_time: sleep time while in critical section
@type sleep_time: C{float}
"""
threading.Thread.__init__(self)
self.__buffer = buffer_
self.__rw_lock = rw_lock
self.__init_sleep_time = init_sleep_time
self.__sleep_time = sleep_time
self.buffer_read = None
"""a copy of a the buffer read while in critical section"""
self.entry_time = None
"""Time of entry to the critical section"""
self.exit_time = None
"""Time of exit from the critical section"""
def run(self):
time.sleep(self.__init_sleep_time)
self.__rw_lock.reader_acquire()
self.entry_time = time.time()
time.sleep(self.__sleep_time)
self.buffer_read = copy.deepcopy(self.__buffer)
self.exit_time = time.time()
self.__rw_lock.reader_release()
class RWLockTestCase(unittest.TestCase):
def test_readers_nonexclusive_access(self):
(buffer_, rw_lock, threads) = self.__init_variables()
threads.append(Reader(buffer_, rw_lock, 0, 0))
threads.append(Writer(buffer_, rw_lock, 0.2, 0.4, 1))
threads.append(Reader(buffer_, rw_lock, 0.3, 0.3))
threads.append(Reader(buffer_, rw_lock, 0.5, 0))
self.__start_and_join_threads(threads)
## The third reader should enter after the second one but it should
## exit before the second one exits
## (i.e. the readers should be in the critical section
## at the same time)
self.assertEqual([], threads[0].buffer_read)
self.assertEqual([1], threads[2].buffer_read)
self.assertEqual([1], threads[3].buffer_read)
self.assert_(threads[1].exit_time <= threads[2].entry_time)
self.assert_(threads[2].entry_time <= threads[3].entry_time)
self.assert_(threads[3].exit_time < threads[2].exit_time)
def test_writers_exclusive_access(self):
(buffer_, rw_lock, threads) = self.__init_variables()
threads.append(Writer(buffer_, rw_lock, 0, 0.4, 1))
threads.append(Writer(buffer_, rw_lock, 0.1, 0, 2))
threads.append(Reader(buffer_, rw_lock, 0.2, 0))
self.__start_and_join_threads(threads)
## The second writer should wait for the first one to exit
self.assertEqual([1, 2], threads[2].buffer_read)
self.assert_(threads[0].exit_time <= threads[1].entry_time)
self.assert_(threads[1].exit_time <= threads[2].exit_time)
def test_writer_priority(self):
(buffer_, rw_lock, threads) = self.__init_variables()
threads.append(Writer(buffer_, rw_lock, 0, 0, 1))
threads.append(Reader(buffer_, rw_lock, 0.1, 0.4))
threads.append(Writer(buffer_, rw_lock, 0.2, 0, 2))
threads.append(Reader(buffer_, rw_lock, 0.3, 0))
threads.append(Reader(buffer_, rw_lock, 0.3, 0))
self.__start_and_join_threads(threads)
## The second writer should go before the second and the third reader
self.assertEqual([1], threads[1].buffer_read)
self.assertEqual([1, 2], threads[3].buffer_read)
self.assertEqual([1, 2], threads[4].buffer_read)
self.assert_(threads[0].exit_time < threads[1].entry_time)
self.assert_(threads[1].exit_time <= threads[2].entry_time)
self.assert_(threads[2].exit_time <= threads[3].entry_time)
self.assert_(threads[2].exit_time <= threads[4].entry_time)
def test_many_writers_priority(self):
(buffer_, rw_lock, threads) = self.__init_variables()
threads.append(Writer(buffer_, rw_lock, 0, 0, 1))
threads.append(Reader(buffer_, rw_lock, 0.1, 0.6))
threads.append(Writer(buffer_, rw_lock, 0.2, 0.1, 2))
threads.append(Reader(buffer_, rw_lock, 0.3, 0))
threads.append(Reader(buffer_, rw_lock, 0.4, 0))
threads.append(Writer(buffer_, rw_lock, 0.5, 0.1, 3))
self.__start_and_join_threads(threads)
## The two last writers should go first -- after the first reader and
## before the second and the third reader
self.assertEqual([1], threads[1].buffer_read)
self.assertEqual([1, 2, 3], threads[3].buffer_read)
self.assertEqual([1, 2, 3], threads[4].buffer_read)
self.assert_(threads[0].exit_time < threads[1].entry_time)
self.assert_(threads[1].exit_time <= threads[2].entry_time)
self.assert_(threads[1].exit_time <= threads[5].entry_time)
self.assert_(threads[2].exit_time <= threads[3].entry_time)
self.assert_(threads[2].exit_time <= threads[4].entry_time)
self.assert_(threads[5].exit_time <= threads[3].entry_time)
self.assert_(threads[5].exit_time <= threads[4].entry_time)
@staticmethod
def __init_variables():
buffer_ = []
rw_lock = RWLock()
threads = []
return (buffer_, rw_lock, threads)
@staticmethod
def __start_and_join_threads(threads):
for t in threads:
t.start()
for t in threads:
t.join()
|
The implementation is quite straightforward and follows cited sources (see the comments in the code). It uses only threading.Lock as a synchronization primitive; it doesn't use threading.Event.
