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SubjectRe: frlock and barrier discussion
Stephen wrote:

[snip - memory barrier for fr_write_begin]

>Using mb() is more paranoid than necessary.


What about the memory barrier in fr_read_begin?
If I understand the Intel documentation correctly, then i386 doesn't need them:
"Writes by a single processor are observed in the same order by all processors"

I think "smp_read_barrier_depends()" (i.e. a nop for i386) is sufficient. Attached is a test app - could someone try it? I don't have access to a SMP system right now.


What about permitting arch overrides for the memory barriers? E.g. ia64 has acquire and release memory barriers - it doesn't map to the Linux wmb()/rmb() scheme.

--
Manfred












/*
* frlock: test for Intel memory ordering.
* Copyright (C) 1999,2003 by Manfred Spraul.
*
* Redistribution of this file is permitted under the terms of the GNU
* Public License (GPL)
* $Header: /pub/home/manfred/cvs-tree/movopt/frlock.cpp,v 1.2 2003/01/26 10:41:39 manfred Exp $
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <assert.h>
static volatile int g_val1;
static volatile int g_val2;
static volatile int g_seq1;
static volatile int g_seq2;

static volatile int start;
#define MB() __asm__ __volatile__ ("lock;addl $0,(%%esp)\n\t" \
:/* no output*/ \
:/* no input*/:"cc","memory")
#define DELAY() do { int i; for(i=0;i<1000;i++); } while(0)
void* threadfnc(void* param)
{
while(!start);
if(1 == (int)param)
goto cpu1;
if(2 == (int)param)
goto cpu2;
assert(0);
cpu1:
{ // reader:
for(;;) {
int x1,x2,val1,val2;
x1 = g_seq1;
val1 = g_val1;
val2 = g_val2;
x2 = g_seq2;
if (x1 == x2) {
if (val1 != val2) {
printf("Bad! memory ordering violation with %d/%d: %d/%d.\n", x1, x2, val1, val2);
}
}
}
}
cpu2:
{ // writer:
int target = 0;
for (;;) {
// write 1:
target++;
g_seq1 = target;
g_val1 = target;
g_val2 = target;
g_seq2 = target;
DELAY();
// write 2:
target++;
g_seq1 = target;
g_val1 = target;
MB();
g_val2 = target;
g_seq2 = target;
DELAY();
// write 3:
target++;
g_seq1 = target;
g_val2 = target;
g_val1 = target;
g_seq2 = target;
DELAY();
// write 4:
target++;
g_seq1 = target;
g_val2 = target;
MB();
g_val1 = target;
g_seq2 = target;
DELAY();


// write 5:
target++;
g_seq1 = target;
g_val1 = target;
MB(); MB();
g_val2 = target;
g_seq2 = target;
DELAY();
// write 6:
target++;
g_seq1 = target;
g_val1 = target;
MB(); DELAY();
g_val2 = target;
g_seq2 = target;
DELAY();
// write 7:
target++;
g_seq1 = target;
g_val2 = target;
MB(); MB();
g_val1 = target;
g_seq2 = target;
DELAY();
// write 8:
target++;
g_seq1 = target;
g_val2 = target;
MB(); DELAY();
g_val1 = target;
g_seq2 = target;
DELAY();
}
}
}
void start_thread(int id)
{
pthread_t thread;
int res;
res = pthread_create(&thread,NULL,threadfnc,(void*)id);
if(res != 0)
assert(false);
}


int main()
{
printf("movopt:\n");
start_thread(1);
start_thread(2);
printf(" starting, please wait.\n");
fflush(stdout);
start = 1;
for(;;) sleep(1000);
}
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