Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

[Atsushi Nemoto <anemo@mba.ocn.ne.jp>]

On Fri, 20 Oct 2006 22:41:22 +0100, Ralf Baechle <ralf@linux-mips.org> wrote:
> > It's just that we weren't quite careful enough at that time (and even 
> > then, that would only matter for some really really unlikely and strange 
> > situations that only happen when you fork() from a _threaded_ environment, 
> > so it shouldn't be anything you'd notice under normal load).
> > 
> > I think.
> 
> The flush is there since a very long time.  I have it in my tree since
> ~ 2.1.36 and I get the feeling anybody every has been seriously revisited
> the issue since.

I think calling fork() (or system() or popen() or so) in threaded
program is neither very unlikely or strange.  But this breakage happens
very rarely indeed, especially non-preemptive kernel.

During debugging this issue, I had used this test program and slightly
modified kernel --- inserting yield() at middle of dup_mmap().

With the modified kernel on 32KB VIPT D$, running this test program
some times could reproduce the breakage ("BAD!" messages).  I heard
PARISC people had successed to reproduce it too.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

static void *thread_func(void *arg)
{
	unsigned char buf[2048], j;
	int i;
	for (j = 0; ; j++) {
		/* fill buf[] with j */
		memset(buf, j, sizeof(buf)/2);
		sched_yield();
		memset(buf + sizeof(buf)/2, j, sizeof(buf)/2);
		sched_yield();
		/* check buf[] contents */
		for (i = 0; i < sizeof(buf); i++) {
			if (buf[i] != j) {
				printf("BAD! %p (%d != %d)\n",
				       buf + i, buf[i], j);
				exit(1);
			}
		}
	}
}

int main(int argc, char *argv[])
{
	int i;
	pid_t pid;
	pthread_t tid;
	for (i = 0; i < 4; i++)
		pthread_create(&tid, NULL, thread_func, NULL);
	for (i = 0; i < 100; i++) {
		pid = fork();
		if (pid == -1) {
			perror("fork");
			exit(1);
		}
		if (pid)
			waitpid(pid, NULL, 0);
		else
			exit(0);
	}
	return 0;
}

---
Atsushi Nemoto
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