= 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 two types of spin-lock:
 * 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.
 * 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 

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

Both the '''spin_lock_t''' and '''simple_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.


 === 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

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

 === void '''_simple_lock_acquire'''( simple_lock_t * lock ) ===
This blocking function does not implement any ordered allocation. 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().