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51 lines
1.7 KiB
51 lines
1.7 KiB
"""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 1 |
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else: |
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return 0 |
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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[0] |
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del self.queue[0] |
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function(argument) |
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else: |
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self.locked = 0
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