determine when a thread is waiting for a lock to release

Question

The lock mechanism (among others) exists to prevent access to shared resources at the same time by different threads. The thread that wants to use the resource is blocked, and must wait for lock to release in order to continue. (Correct me if I'm wrong)

How can I tell when a thread is waiting for a lock to be released, and when not? In other words, how to know when a thread is blocked?.

If this is possible, how can I tell how many times this occurs?

Answer 1

You can use the optional blocking argument to the Lock.acquire() function, if you provide False for blocking like Lock.acquire(False), the function will return False if the lock cannot be acquired (held by another thread) or it will acquire the lock and return True. You can use this with the following logic to determine if your thread will be blocking:

import threading
some_lock = threading.Lock()

# use this code anywhere you need to acquire the lock and check for blocking
if not some_lock.acquire(False):
    print 'BLOCKING!'
    some_lock.acquire() # call again with blocking=True (which is the default)
else:
    print 'Acquired lock without blocking'
# lock has been acquired, make sure you release when done with critical section

The print statements are just there to show what is going on, replace this with whatever logic you want to use, for example maintaining a counter of times the thread blocked.