GIET-VM / Locks access functions

The locks.c and 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 )