From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Tue, 31 Aug 2010 08:54:49 -0400
To: 9fans@9fans.net
Message-ID: <4fdbe45db39b4c9d8bb6f360e12dde9c@brasstown.quanstro.net>
In-Reply-To: <AANLkTikVuCtt-CJH5C30L_1iUw=ioDpuUaWu4-jNjH9N@mail.gmail.com>
References: <AANLkTikVuCtt-CJH5C30L_1iUw=ioDpuUaWu4-jNjH9N@mail.gmail.com>
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Subject: Re: [9fans] problem with building from source on vx32 solved
Topicbox-Message-UUID: 4b1a0e30-ead6-11e9-9d60-3106f5b1d025

> A simple change:
> pid = getpid();

shouldn't you just fix it so _tos->pid is set?

also, the comments in the new nsec.c don't
match the code.  the code looks something like this

	do{
		...
		tries = 0;
	}while(tries++ == 0);	/* retry once */

of course either this should be

	/* just retry for ever */
	for(;;){
		...
	}
or
	/* retry once */
	for(tries = 0; tries < 2; tries++)

also, there's nothing stoping the race between
- two procs allocating the same element of the pid array
- two procs opening the global fd twice

both cases have the potential to leak an unlimited amount
of file descriptors.  i've added a proposed change.

- erik

----
#include <u.h>
#include <libc.h>
#include <tos.h>

static uvlong order = 0x0001020304050607ULL;

static void
be2vlong(vlong *to, uchar *f)
{
	uchar *t, *o;
	int i;

	t = (uchar*)to;
	o = (uchar*)&order;
	for(i = 0; i < sizeof order; i++)
		t[o[i]] = f[i];
}

static Lock nseclk;
static Lock fdlk;
static int fd = -1;
static struct {
	int	pid;
	int	fd;
} fds[64];

vlong
nsec(void)
{
	uchar b[8];
	vlong t;
	int pid, i, *f, tries;

	/*
	 * Threaded programs may have multiple procs
	 * with different fd tables, so we may need to open
	 * /dev/bintime on a per-pid basis
	 */
	pid = _tos->pid;
	f = nil;
	for(i = 0; i < nelem(fds); i++)
		if(fds[i].pid == pid){
			f = &fds[i].fd;
			break;
		}
	if(i == nelem(fds)){
		lock(&nseclk);
		for(i = 0; i < nelem(fds); i++)
			if(fds[i].pid == 0){
				fds[i].pid = pid;
				fds[i].fd = -1;
				f = &fds[i].fd;
				break;
			}
		unlock(&nseclk);
	}
	if(f == nil)
		f = &fd;
	for(tries = 0; tries < 2; tries++){
		if(*f < 0){
			if(f == &fd)
				lock(&fdlk);
			if((*f = open("/dev/bintime", OREAD|OCEXEC)) < 0)
				break;
			fd = *f;
			if(f == &fd)
				unlock(&fdlk);
		}
		if(pread(*f, b, sizeof b, 0) == sizeof b){
			be2vlong(&t, b);
			return t;
		}
		close(*f);
		*f = -1;
	}
	if(i < nelem(fds))
		fds[i].pid = 0;	/* unlocked release */
	return 0;
}