Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Johannes Stezenbach
2005-07-14T20:38:05Z

On Thu, Jul 14, 2005 Linus Torvalds wrote:
> In other words, the _right_ way to do this is literally
>
> unsigned long timeout = jiffies + HZ/2;
> for (;;) {
> if (ready())
> return 0;
> if (time_after(timeout, jiffies))
> break;
> msleep(10);
> }
>
> which is unquestionably more complex, yes, but it's more complex because
> it is CORRECT!

Since you emphasised on correctness, your code is actually buggy
for the preemptible kernel. It could get preempted after the ready() test,
but before the time_after(), for quite a whie if a high priority process
keeps the system busy. This code is better:

unsigned long timeout = jiffies + HZ/2;
int err;
for (;;) {
err = time_after(timeout, jiffies);
if (ready())
return 0;
if (err)
break;
msleep(10);
}

This way the condition is always re-tested before reporting timeout.

Johannes