= GIET-VM / Locks access functions =

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The [source:soft/giet_vm/giet_common/locks.c locks.c] and [source:soft/giet_vm/giet_common/locks.h locks.h] files define the functions used by the kernel to take & release  locks protecting exclusive access to a shared resource. 

The GIET_VM kernel define three types of spin-lock:

 1. The '''simple_lock_t''' implements a spin-lock without waiting queue. It is only used by the TTY0 access functions, to get exclusive access to the TTY0 kernel terminal.

 2. The '''spin_lock_t''' implements a spin-lock with a waiting queue (based on a ticket allocator scheme), to enforce fairness and avoid live-lock situations. The GIET_LOCK_MAX_TICKET define the wrapping value for the ticket allocator. This lock
must be initialised.

 3. The '''sbt_lock_t''' spin-lock can be used when a single lock protect a unique resource that can be used by any task
running on a 2D mesh of clusters. The lock is implemented as a Sliced Binary Tree of ''partial'' locks distributed on all cluster
of a 2D mesh. It is intended to avoid contention on a single cluster when all tasks try to access the same lock.

All the lock access functions are prefixed by "_" to remind that they can only be executed by a processor in kernel mode.

The '''simple_lock_t''', '''sbt_lock_t''', and '''spin_lock_t''' structures are implemented to have one single lock in a 64 bytes cache line, and should be aligned on a cache line boundary.

== Atomic access functions ==

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

== Simple lock access functions ==

 === void '''_simple_lock_acquire'''( simple_lock_t * lock ) ===
This blocking function does not implement any ordered allocation, and is not distributed. It returns only when the lock as been granted.

 === void '''_simple_lock_release'''( simple_lock_t * lock ) ===
This function releases the lock, and can be used for lock initialisation. It must always be called after a successful _simple_lock_acquire().

== Queuing Lock Access functions == 

 === void '''_lock_init'''( spin_lock_t * lock ) ===
This function initializes the lock ticket allocator.

 === void '''_lock_acquire'''( spin_lock_t * lock ) ===
This blocking function uses the atomic_increment() function, to implement a ticket allocator and provide ordered access to the protected resource. It returns only when the lock as been granted.

 === void '''_lock_release'''( spin_lock_t * lock ) ===
This function releases the lock, but cannot be used for lock initialisation. It must always be called after a successful _lock_acquire().

== Distributed Lock Access functions ==

 === void '''sbt_lock_init'''( sbt_lock_t*   lock,  unsigned int   ntasks ) === 

 === void '''sbt_lock_acquire'''( sbt_lock_t*  lock ) ===

 === void '''sbt_lock_release'''( sbt_lock_t*  lock ) ===