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    SubjectRe: testing/pre-7 and do_poll()


    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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