The user_lock library

The user_lock.c​ and user_lock.h​ files define two types of services:

• user-level atomic access functions, to increment a shared counter.
• user-level spin-locks, to obtain exclusive access to a shared resource in a parallel multi-tasks application.

The proposed spin-lock implement a waiting queue service, to enforce fairness and avoid live-lock situations. Each lock (giet_lock_t object) occupies a complete 64 bytes cache line to avoid false sharing.

Lock placement

The lock being shared by several tasks, should be defined as a global variable of type user_lock_t in the application code. It is possible to control precisely the placement of a specific lock by defining a specific vseg in the application mapping.

Access functions

unsigned int atomic_increment( unsigned int* shared, unsigned int increment)

This function uses a LL/SC to atomically increment a shared variable.

• shared : pointer on the shared variable
• increment : increment value.

void lock_init( user_lock_t * lock )

This function should be called by one single task.

• lock : pointer on the user_lock_t structure.

void lock_acquire( giet_lock_t * lock )

This blocking function returns only when the lock has been successfully taken.

• lock : pointer on the user_lock_t structure.

void lock_release( giet_lock_t * lock )

This function releases the lock. It must be called by a task after a successful lock_acquire().

• lock : pointer on the user_lock_t structure.