[Stackless] mutex class containing tool(s) for preemptive in SLP3.0

Eric van Riet Paap eric2 at vanrietpaap.com
Sun Apr 27 12:17:03 CEST 2003

Hi All,

I've created this small piece of code (that I can't test yet) to provide the 
preemptive multitasking with stackless python that Chris is currently working 

Comments/hints are very welcome: 

(file: preemptive.py)

'''Provide tools for preemptive multitasking with stackless python.

Actual scheduling/locking is performed by communicating over (stackless') 
The goal here is to set the atomic flag for a short a period as possible.

Author: Eric van Riet Paap <eric at vanrietpaap.com>'''
import stackless

class mutex:
        '''mutual exclusion object'''

        data = {}

        def __init__(self, name):
                '''mutexes are initialy unlocked'''
                self.name = name
                if not self.channel.has_key(name):
                        self.data[name] = [0, stackless.channel()]

        def isLocked(self):
                '''return non-zero if locked'''
                return self.data[self.name][0]

        def lock(self):
                '''acquire the lock'''
                a = stackless.setatomic(1)
                n, ch = self.data[self.name]
                self.data[self.name][0] = n+1
                if n: #other tasklet acquired the mutex
                        #note: we actually might need to setatomic(0) here, 
however I'm a bit afraid of switches that
                        #      might happen between setatomic(0) and the next 
line! (Chris, what do you think?)
                        ch.receive()    #wait for tasklet with lock to send 

        def unlock(self):
                '''release the lock'''
                a = stackless.setatomic(1)
                n, ch = self.data[self.name]
                self.data[self.name][0] = n-1
                if n: #other tasklets would like to aqcuire the mutex
                        #note: same note is with lock method.
                        ch.send(n)      #wake up a tasklets that is trying to 
receive data from this channel

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