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#include <u.h>
#include <libc.h>
static long lockinit(long);
/*
* barrier is called from atom.s and tas.s assembly
* to execute memory barrier.
*/
long (*_barrier)(long) = lockinit;
static int
cpus(void)
{
char buf[256], *p;
int f, n;
f = open("#c/sysstat", OREAD|OCEXEC);
if(f < 0)
return -1;
n = read(f, buf, sizeof(buf)-1);
close(f);
if(n <= 0)
return -1;
buf[n] = '\0';
n = 0;
p = buf;
while(*p != '\0'){
if(*p == '\n')
n++;
p++;
}
return n;
}
long _dmb(long);
static long
_nop(long r0)
{
return r0;
}
static long
lockinit(long r0)
{
if(cpus() > 1)
_barrier = _dmb;
else
_barrier = _nop;
return (*_barrier)(r0);
}
void
lock(Lock *lk)
{
int i;
/* once fast */
if(!_tas(&lk->val))
return;
/* a thousand times pretty fast */
for(i=0; i<1000; i++){
if(!_tas(&lk->val))
return;
sleep(0);
}
/* now nice and slow */
for(i=0; i<1000; i++){
if(!_tas(&lk->val))
return;
sleep(100);
}
/* take your time */
while(_tas(&lk->val))
sleep(1000);
}
int
canlock(Lock *lk)
{
return _tas(&lk->val) == 0;
}
void
unlock(Lock *lk)
{
lk->val = (*_barrier)(0);
}
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