Re: testing/pre-7 and do_poll()

[Linus Torvalds <torvalds@transmeta.com>]

On Sun, 10 Jan 1999, Chip Salzenberg wrote:
> 
> Got it, like it -- *except* the fix for overflow in do_poll() is a
> little bit off.  Quoting testing/pre-7:
> 
> 	if (timeout) {
> 		/* Carefula about overflow in the intermediate values */
> 		if ((unsigned long) timeout < MAX_SCHEDULE_TIMEOUT / HZ)
> 			timeout = (timeout*HZ+999)/1000+1;
> 		else /* Negative or overflow */
> 			timeout = MAX_SCHEDULE_TIMEOUT;
> 	}
> 
> However, the maximum legal millisecond timeout isn't (as shown)
> MAX_SCHEDULE_TIMEOUT/HZ, but rather MAX_SCHEDULE_TIMEOUT/(1000/HZ).
> So this code will turn some large timeouts into MAX_SCHEDULE_TIMEOUT
> unnecessarily.

Note the comment (and do NOT look at the speeling).

In particular, we need to make sure the _intermediate_ value doesn'
toverflow. We could do that by using 64-bit arithmetic, but let's not.

> ! 	if (timeout < 0)
> ! 		timeout = MAX_SCHEDULE_TIMEOUT;
> ! 	else if (timeout)
> ! 		timeout = ROUND_UP(timeout, 1000/HZ);

Eh? And re-introduce the original bug?

		Linus

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