Messages in this thread | | | Date | Sun, 10 Jan 1999 22:02:47 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: testing/pre-7 and do_poll() |
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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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