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51 lines
1.7 KiB
51 lines
1.7 KiB
14 years ago
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"""Mutual exclusion -- for use with module sched
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A mutex has two pieces of state -- a 'locked' bit and a queue.
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When the mutex is not locked, the queue is empty.
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Otherwise, the queue contains 0 or more (function, argument) pairs
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representing functions (or methods) waiting to acquire the lock.
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When the mutex is unlocked while the queue is not empty,
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the first queue entry is removed and its function(argument) pair called,
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implying it now has the lock.
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Of course, no multi-threading is implied -- hence the funny interface
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for lock, where a function is called once the lock is aquired.
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"""
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class mutex:
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def __init__(self):
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"""Create a new mutex -- initially unlocked."""
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self.locked = 0
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self.queue = []
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def test(self):
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"""Test the locked bit of the mutex."""
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return self.locked
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def testandset(self):
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"""Atomic test-and-set -- grab the lock if it is not set,
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return True if it succeeded."""
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if not self.locked:
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self.locked = 1
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return True
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else:
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return False
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def lock(self, function, argument):
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"""Lock a mutex, call the function with supplied argument
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when it is acquired. If the mutex is already locked, place
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function and argument in the queue."""
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if self.testandset():
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function(argument)
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else:
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self.queue.append((function, argument))
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def unlock(self):
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"""Unlock a mutex. If the queue is not empty, call the next
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function with its argument."""
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if self.queue:
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function, argument = self.queue.pop(0)
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function(argument)
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else:
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self.locked = 0
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