= GIET-VM / Locks access functions =

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

The GIET_VM kernel define three types of locks:

 1. The '''simple_lock_t''' implements a non-distributed spin-lock without waiting queue. 

 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. 

 3. The '''sqt_lock_t''' spin-lock can be used when a single lock protect a unique resource shared by a large number of tasks running on a 2D mesh clusterised architecture. The lock is implemented as a Synchronisation Quad Tree (SQT) of ''partial'' spin_locks distributed on all cluster, and is intended to avoid contention on a single cluster when all tasks try to access the same resource.

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 function ==

 === 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
It returns the value of the shared variable before increment.

== 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 '''_spin_lock_init'''( spin_lock_t * lock ) ===
This function initializes the spin_lock ticket allocator.

 === void '''_spin_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 lock. It returns only when the lock as been granted.

 === void '''_spin_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 ) === 
This function allocates and initialises the SQT nodes, distributed on all clusters. It computes the smallest SQT covering all processors defined in the mapping. This function use the _remote_malloc() function, and the distributed kernel heap segments.

 === void '''_sbt_lock_acquire'''( sbt_lock_t*  lock ) ===
This function tries to acquire the SBT lock: It tries to get each "partial" lock on the path from bottom to top, using an atomic LL/SC, and starting from bottom. It is blocking : it polls each "partial" lock until it can be taken, and returns only  when all "partial" locks, at all levels have been obtained.

 === void '''_sbt_lock_release'''( sbt_lock_t*  lock ) ===
This function releases the SBT lock: It reset all "partial" locks on the path from bottom to top, using a normal write, and starting from bottom.