int l4_mutex_control(void *mutex_word, int op);

• Userspace locking and synchronization for shared memory

• op argument defines whether the operation is a lock or unlock operation.
• mutex_word argument specifies the userspace virtual address for the lock.
  • Typically the lock is placed on a shared memory segment.
  • The virtual address can be different on different address spaces referring to the same lock.
• Mutexes are locked and unlocked with no system call assistance if there is no lock contention.
• If a lock contention occurs, lock holder and lock contender synchronize by the assistance of this system call.
• In its current state this system call represents a *binary semaphore* implementation.

 
#define L4_MUTEX_LOCK          0
#define L4_MUTEX_UNLOCK		1
 
struct l4_mutex {
	unsigned int lock;
} __attribute__((aligned(sizeof(int))));

void l4_mutex_init(struct l4_mutex *m);
int l4_mutex_lock(struct l4_mutex *m);
int l4_mutex_unlock(struct l4_mutex *m);

• l4_mutex_init() initializes an uninitialized mutex.
• l4_mutex_lock() locks an initialized, mutex. If it contends, it calls the mutex syscall.
• l4_mutex_unlock() releases an acquired mutex. If there was contention, mutex syscall is called to resolve by the kernel.

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