= The user_lock library =

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The [source:soft/giet_vm/giet_libs/user_lock.c user_lock.c] and [source:soft/giet_vm/giet_libs/user_lock.h user_lock.h] 
files define two types of locks: 
 * '''user_lock_t''' : this lock uses a ticket allocation mechanism to enforce fairness and avoid live-locks. This lock is located in one single cluster, and the placement can be controlled by defining a specific vseg in the application mapping. 
 * '''sqt_lock_t''' : this  distributed lock can be used to avoid quadratic latency, when the protected resource is shared by a large number or threads. The SQT (Synchronization Quad-Tree) is distributed on all clusters of the mesh.

Each lock (user_lock_t object, or distributed sqt_lock_node_t) occupies a complete 64 bytes cache line to avoid false sharing.

The '''user_lock_t''' and '''sqt_lock_t''' being shared by several threads, should be defined as a global variable in the application code.

 == Atomic access functions ==

Both types of locks use the atomic_increment() function, that can be directly used by the applications.

 === unsigned int '''atomic_increment'''( unsigned int* shared, unsigned int increment) ===
This blocking function uses a LL/SC to atomically increment a shared variable.
 * '''ptr''' : pointer on the shared variable
 * '''increment''' : increment value.

 == Simple lock access functions

 === '''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'''( user_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'''( user_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.

 == Distributed lock access functions ==

The SQT topology is completely defined by the (x_size / y_size) parameters, with the following constraints:
 * The involved clusters form a mesh(x_size * y_size).
 * The lower left involved cluster is cluster(0,0).
 * All involved clusters must contain a heap_x_y vseg declared in the mapping.
 * The number of involved tasks in a given cluster is the same for all involved clusters. 

 === '''void sqt_lock_init'''( sqt_lock_t * lock ) ===
This function should be called by one single task.
 * '''lock''' : pointer on the sqt_lock_t structure.

 === '''void sqt_lock_acquire'''( sqt_lock_t * lock ) ===
This blocking function returns only when all locks of the distributed tree have been successfully taken.
 * '''lock''' : pointer on the sqt_lock_t structure.

 === '''void sqt_lock_release'''( sqt_lock_t * lock ) ===
This function releases the lock. It must be called by a task after a successful lock_acquire().
 * '''lock''' : pointer on the sqt_lock_t structure.