What are the different types of locks in multithreading?

1 What are locks?

Locks are objects that have two states: locked and unlocked. A thread that wants to access or modify a shared data or resource must first acquire the lock associated with it, which means changing its state from unlocked to locked. While the lock is locked, no other thread can acquire it, and must wait until the lock is released by the thread that holds it, which means changing its state back to unlocked. This way, locks ensure that only one thread at a time can access or modify a shared data or resource, and prevent race conditions and data corruption.

2 Types of locks

Different types of locks vary in design, implementation, and behavior, such as a mutex (short for mutual exclusion), which allows only one thread to acquire it at a time and can be either recursive or non-recursive. A semaphore is a lock with a counter that indicates how many threads can acquire it at a time, and can be either binary or counting. A spinlock does not block the thread that tries to acquire it, but instead makes it busy-wait in a loop until the lock becomes available. Lastly, a read-write lock allows multiple threads to acquire it for reading, but only one thread to acquire it for writing. Such locks can improve the concurrency and throughput of applications that have more read operations than write operations.

3 How to use locks

To use locks effectively and correctly, there are some best practices and guidelines to follow. First, you should choose the appropriate type of lock for the scenario and data structure, such as a mutex for a critical section or a read-write lock for a shared data structure. Second, acquire and release locks in a consistent order and scope to prevent deadlocks or blocking. Third, avoid nested locks and lock contention. For instance, if one thread acquires a lock L1 and then tries to acquire another lock L2 held by another thread, it can cause a deadlock. This can be avoided by using recursive locks or redesigning the logic. Lastly, lock contention can be reduced by using finer-grained locks or reducing the frequency or duration of lock acquisitions.

4 Benefits and drawbacks of locks

Locks are essential for ensuring the correctness and consistency of multithreaded applications. While they can prevent race conditions, data corruption, and inconsistent results, they can also introduce overhead, complexity, and potential errors to the application. In terms of memory, CPU, and time, locks require extra space, instructions, and context switches. Moreover, they require careful design, implementation, and testing to avoid common errors such as deadlocks, blocking, starvation, and livelocks. On the other hand, locks can improve the performance and efficiency of applications that perform complex or parallel tasks by allowing threads to cooperate and coordinate their work.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?