python - Django Celery cache lock did not work? -

i trying use django cache implement lock mechanism. in celery offical site, claimed django cache work fine this. however, in experence, did not work. experience if there multiple threads/processes acquire lock in same time (close ~0.003 second), threads/processes lock successfully. other threads acquire lock later ~0.003 second, fails.

am person experienced this? please correct me if possible.

def acquire(self, block = false, slp_int = 0.001):     while true:         added = cache.add(self.ln, 'true', self.timeout)          if added:             cache.add(self.ln + '_pid', self.pid, self.timeout)             return true          if block:             sleep(slp_int)             continue         else:             return false   # set django backend cache localcache caches = {      'default': {         'backend': 'django.core.cache.backends.filebased.filebasedcache',         'location': '/dev/shm/django_cache',                                                                                                                                                                                                   }    } 

the problem django makes no guarantees atomicity of .add(). whether or not .add() in fact atomic depends on backend using. filebasedcache, .add() not atomic:

def add(self, key, value, timeout=default_timeout, version=none):     if self.has_key(key, version):         return false     self.set(key, value, timeout, version)     return true 

worker executing .add() preempted after self.has_key(...) before self.set(...). worker b executing .add() in 1 shot set key , return true. when worker resumes, set key , return true.

this issue report indicates example code looked @ assumes backend memcached. if use memcached or backend supports atomic .add() should work.