How can you implement a reader-writer lock?

1 What is a semaphore?

A semaphore is a variable that represents the number of available resources or permissions for a certain operation. It can be used to control access to a critical section, such as a shared resource, by allowing only a limited number of threads to enter at a time. A thread must acquire a semaphore before entering the critical section, and release it after leaving. There are two types of semaphores: binary and counting. A binary semaphore can only have two values: 0 or 1, and is used to implement mutual exclusion. A counting semaphore can have any non-negative value, and is used to implement resource allocation.

2 What is a mutex?

A mutex is a special type of binary semaphore that provides mutual exclusion for a single resource. It has two operations: lock and unlock. A thread must lock a mutex before accessing the resource, and unlock it after finishing. Only one thread can lock a mutex at a time, and the same thread must unlock it. A mutex can also have an owner and a priority, to prevent deadlock and priority inversion.

3 What is a counter?

A counter is a variable that keeps track of the number of threads that are performing a certain action, such as reading or writing. It can be used to implement conditional synchronization, such as waiting for all threads to finish before proceeding. A counter can be incremented or decremented by threads, but it must be protected by a mutex or a semaphore to ensure atomicity.

4 How to implement a reader-writer lock?

A reader-writer lock can be implemented using three components: a read-write semaphore, a read mutex, and a read counter. The read-write semaphore is a binary semaphore that controls the access to the shared resource and is acquired by writers and released by readers. The read mutex is a mutex that protects the read counter, which counts the number of active readers. To read, a thread must lock the read mutex, increment the read counter, acquire the read-write semaphore if it is 1, unlock the read mutex, read the shared resource, lock the read mutex again, decrement the read counter, release the read-write semaphore if it is 0, and unlock the read mutex. To write, a thread must acquire the read-write semaphore, write to the shared resource, and release the read-write semaphore. This algorithm will help inform and protect access to shared resources.

5 What are the advantages and disadvantages of a reader-writer lock?

A reader-writer lock has some advantages and disadvantages compared to other synchronization mechanisms, such as a simple mutex. For example, it allows concurrent reads, which can improve performance and throughput, as well as reduce contention and waiting time for readers. Additionally, it preserves data consistency and integrity for writers. However, it introduces complexity and overhead for managing the semaphores, mutexes, and counters. It can also cause starvation and deadlock for writers if there are too many readers or if they do not release the read-write semaphore promptly. Moreover, it can cause priority inversion for writers if they have higher priority than readers but have to wait for them to finish.

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?