threading.RLock

Motivating a Reentrant Lock

• There will be times when we want to nest locks within locks

• In these scenario, we'll likely:

  • Take one thread inside of a lock
  • Branch off to create more threads
  • Adding another lock for those nested threads
• As a result, we will be nesting locks
• Consequently, this will create a deadlock
• The following steps will deadlock:

>>> from threading import Lock
>>> l = threading.Lock()
>>> l.acquire()
>>> l.acquire()  # acquiring lock twice -> hangs
>>> l.release()
>>> l.release()

• Therefore, the RLock is designed for handling nested locks

Describing a Reentrant Lock

• A reentrant lock is a synchronization primitive that's similar to a Lock object
• However, it can be acquired multiple times by the same thread
• This allows the thread owning the lock to perform nested acquire() and release() operations
• In this case, only the outer-most release() operation resets the lock to its unlocked state

Describing RLock()

• A reentrant lock is represented using the RLock object
• A new RLock object is created using the RLock() constructor
• It creates a new reentrant lock object

• An RLock instance rlock has two methods:

  • rlock.acquire()
  • rlock.release()

Describing rlock.acquire(blocking=True)

• acquire acquires the lock, blocking until the lock is released

• If no thread owns the lock:

  • The thread is locked
  • And the recursion level is set to $1$
• If this thread already owns the lock, the recursion level of the lock is increased by one and the function returns immediately

Describing rlock.release()

• release releases the lock by decrementing its recursion level
• If the recursion level is $0$ after the decrement, then the lock is reset to the unlocked state
• Otherwise, the lock remains locked
• This function should only be called by the thread that currently owns the lock

Example with RLock

>>> from threading import RLock
>>>> l = threading.RLock()
>>> l.acquire()
>>> l.acquire()  # acquiring lock twice -> doesn't hang
>>> l.release()
>>> l.release()

References

• Python Essential References
• Documentation for Threading API
• Examples of Threading in Python
• Difference between Locks and Semaphores
• Example Code using threading.Lock
• Details about Reentrant Locks