87 lines
2.8 KiB
Python
87 lines
2.8 KiB
Python
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# Copyright Mateusz Kobos, (c) 2011
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# https://code.activestate.com/recipes/577803-reader-writer-lock-with-priority-for-writers/
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# released under the MIT licence
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import threading
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__author__ = "Mateusz Kobos"
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class RWLock:
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"""
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Read-Write locking primitive
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Synchronization object used in a solution of so-called second
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readers-writers problem. In this problem, many readers can simultaneously
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access a share, and a writer has an exclusive access to this share.
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Additionally, the following constraints should be met:
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1) no reader should be kept waiting if the share is currently opened for
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reading unless a writer is also waiting for the share,
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2) no writer should be kept waiting for the share longer than absolutely
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necessary.
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The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7]
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with a modification -- adding an additional lock (C{self.__readers_queue})
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-- in accordance with [2].
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Sources:
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[1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008
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[2] P.J. Courtois, F. Heymans, D.L. Parnas:
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"Concurrent Control with 'Readers' and 'Writers'",
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Communications of the ACM, 1971 (via [3])
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[3] http://en.wikipedia.org/wiki/Readers-writers_problem
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"""
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def __init__(self):
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"""
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A lock giving an even higher priority to the writer in certain
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cases (see [2] for a discussion).
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"""
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self.__read_switch = _LightSwitch()
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self.__write_switch = _LightSwitch()
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self.__no_readers = threading.Lock()
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self.__no_writers = threading.Lock()
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self.__readers_queue = threading.Lock()
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def reader_acquire(self):
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self.__readers_queue.acquire()
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self.__no_readers.acquire()
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self.__read_switch.acquire(self.__no_writers)
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self.__no_readers.release()
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self.__readers_queue.release()
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def reader_release(self):
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self.__read_switch.release(self.__no_writers)
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def writer_acquire(self):
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self.__write_switch.acquire(self.__no_readers)
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self.__no_writers.acquire()
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def writer_release(self):
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self.__no_writers.release()
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self.__write_switch.release(self.__no_readers)
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class _LightSwitch:
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"""An auxiliary "light switch"-like object. The first thread turns on the
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"switch", the last one turns it off (see [1, sec. 4.2.2] for details)."""
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def __init__(self):
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self.__counter = 0
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self.__mutex = threading.Lock()
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def acquire(self, lock):
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self.__mutex.acquire()
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self.__counter += 1
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if self.__counter == 1:
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lock.acquire()
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self.__mutex.release()
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def release(self, lock):
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self.__mutex.acquire()
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self.__counter -= 1
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if self.__counter == 0:
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lock.release()
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self.__mutex.release()
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