Question

This article: http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf (page 12)
seems to make a difference between a lock and a memory barrier

I would like to know what the difference is between a lock, memory barrier, and a semaphore?

(While other questions might mention the difference between a lock and a synchronisation object, I found none about the difference between a lock and a memory barrier)

Answer 1

• A memory barrier is a method to order memory access. Compilers and CPU’s can change this order to optimize, but in multithreaded environments, this can be an issue. The main difference with the others is that threads are not stopped by this.
• A lock or mutex makes sure that code can only be accessed by 1 thread. Within this section, you can view the environment as singlethreaded, so memory barriers should not be needed.
• a semaphore is basically a counter that can be increased (v()) or decreased (p()). If the counter is 0, then p() halts the thread until the counter is no longer 0. This is a way to synchronize threads, but I would prefer using mutexes or condition variables (controversial, but that’s my opinion). When the initial counter is 1, then the semaphore is called a binary semaphore and it is similar to a lock.

A big difference between locks and semaphores is that the thread owns the lock, so no other thread should try to unlock, while this is not the case for semaphores.