Re: [9fans] devproc noteid changing for none

[erik quanstrom <quanstro@quanstro.net>]

On Thu Jan  2 17:32:08 EST 2014, quanstro@quanstro.net wrote:
> On Thu Jan  2 16:55:59 EST 2014, cinap_lenrek@felloff.net wrote:
> > i was investigating all callers of postnote() for bugs that could
> > lead to spurious notes like the alarm race i described before. btw
> > this has been fixed too. the key is to recheck p->alarm while holding
> > p->debug qlock. once you have it, the process cannot exit under you.
>
> cool.  if that's the protocol, doesn't the debug lock need to be held whenever
> up->alarm is modified?

checkalarms will hang during rollover.  suppose sys->ticks = 1<<31,
then (long)((1<<31) - 0) = -2147483648.  this will stay negative for a
2147483648/HZ seconds.

i think this is necessary:

void
checkalarms(void)
{
	Proc *p;
	ulong now;

	p = alarms.head;
	now = sys->ticks;

	if(p != nil)
	if(p->alarm == 0 || (long)(now - p->alarm) >= 0)
		wakeup(&alarmr);
}

- erik