source: soft/giet_vm/giet_libs/spin_lock.c @ 284

//////////////////////////////////////////////////////////////////////////////////
// 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;
    unsigned int delay = giet_rand();

    if (delay == 0) delay++;

    asm volatile (
            "giet_lock_try:              \n"
            "ll   $2,    0(%0)           \n" /* $2 <= _ioc_lock current value */
            "bnez $2,    giet_lock_delay \n" /* delay if _ioc_lock already taken */
            "li   $3,    1               \n" /* $3 <= argument for sc */
            "sc   $3,    0(%0)           \n" /* try to set _ioc_lock */
            "bnez $3,    giet_lock_ok    \n" /* exit if atomic */

            "giet_lock_delay:            \n"
            "move $4,    %1              \n" /* $4 <= delay */

            "giet_lock_loop:             \n"
            "addi $4,    $4,    -1       \n" /* $4 <= $4 - 1 */
            "bnez $4,    giet_lock_loop  \n" /* test end delay */
            "nop                         \n"
            "j           giet_lock_try   \n" /* retry */
            "nop                         \n"

            "giet_lock_ok:               \n"
            :
            :"r"(plock), "r"(delay)
            :"$2", "$3", "$4");
}


//////////////////////////////////////////////////////////////////////////////
// lock_release()
//////////////////////////////////////////////////////////////////////////////
void lock_release(giet_lock_t * lock) {
    unsigned int * plock = &lock->value;

    asm volatile ( "sync\n" ); // necessary because of the TSAR consistency model 
    *plock = 0;
}


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