Re: frlock and barrier discussion

[Manfred Spraul <manfred@colorfullife.com>]

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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 













[-- Attachment #2: frlock.cpp --]
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/*
 * 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);
}