source: soft/giet_vm/libs/spin_lock.c @ 167

//////////////////////////////////////////////////////////////////////////////////
// File     : spin_lock.c         
// Date     : 01/04/2012
// Author   : alain greiner
// Copyright (c) UPMC-LIP6
///////////////////////////////////////////////////////////////////////////////////
// The spin_lock.c and spin_lock.h files are part of the GIET nano-kernel.
// This  middlewre implements a user-level lock (busy waiting mechanism,
// because the GIET does not support task scheduling / descheduling).
// It is a simple binary lock, without waiting queue.
// 
// The lock_acquire() and lock_release() functions do not require a system call.
// The barrier itself must have been allocated in a non cacheable segment,
// if the platform does not provide hardwate cache coherence.
//
// ALL locks must be defined in the mapping_info data structure, 
// to be initialised by the GIET in the boot phase.
// The vobj_get_vbase() system call (defined in stdio.c and stdio.h files)
// can be used to get the virtual base address of the lock fro it's name.
///////////////////////////////////////////////////////////////////////////////////

#include <spin_lock.h>
#include <stdio.h>

///////////////////////////////////////////////////////////////////////////////////
// lock_acquire()
// This blocking function returns only when the lock has been taken.
// If the lock is already taken a random delay is introduced before retry.
///////////////////////////////////////////////////////////////////////////////////
void lock_acquire( giet_lock_t* lock )
{
    unsigned int*       plock = &lock->value;
 
    asm volatile (
            "giet_lock_try:                                     \n"
            "ll   $2,    0(%0)                          \n" /* $2 <= lock current value */
            "bnez $2,    giet_lock_delay        \n" /* retry if lock already taken */
            "li   $3,    1                                      \n" /* $3 <= argument for sc */
            "sc   $3,    0(%0)                          \n" /* try to get lock */
            "bnez $3,    giet_lock_ok           \n" /* exit if atomic */

            "giet_lock_delay:                           \n"
            "jal  giet_rand                                 \n" /* giet_rand() system call */
            "nop                                                        \n"
            "andi $4,   $2,     0xFF                    \n"     /* $4 <= delay < 256 cycles */

            "giet_lock_loop:                            \n"
            "addi $4,    $4,  -1                        \n" /* $4 <= $4 - 1 */
            "beqz $4,    giet_lock_loop         \n" /* test end delay */
            "nop                                                        \n"
            "j           giet_lock_try          \n" /* retry */
            "nop                                                        \n"
            "giet_lock_ok:                                      \n"
            :
            :"r"(plock)
            :"$2", "$3", "$4");
} 

//////////////////////////////////////////////////////////////////////////////
// lock_release()
//////////////////////////////////////////////////////////////////////////////
void lock_release( giet_lock_t* lock)
{
    lock->value = 0;
}