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    SubjectRe: [PATCH 1/4] fs/autofs: Use time_before, time_before_eq, etc.
    On Wed, 26 Dec 2007, H. Peter Anvin wrote:

    > Ray Lee wrote:
    > > On Dec 26, 2007 7:21 AM, Julia Lawall <julia@diku.dk> wrote:
    > > > - if (jiffies - ent->last_usage < timeout)
    > > > + if (time_before(jiffies, ent->last_usage + timeout))
    > >
    > > I don't think this is a safe change? subtraction is always safe (if
    > > you think about it as 'distance'), addition isn't always safe unless
    > > you know the range. The time_before macro will expand that out to
    > > (effectively):
    > >
    > > if ( (long)(ent->last_usage + timeout) - (long)(jiffies) < 0 )
    > >
    > > which seems to introduce an overflow condition in the first term.
    > >
    > > Dunno, I may be wrong (happens often), but at the very least what
    > > you've transformed it into is no longer obviously correct, and so it's
    > > not a great change.
    >
    > Indeed. The bottom form will have overflow issues at time
    > jiffies_wraparound/2, whereas the top form will have overflow issues only near
    > jiffies_wraparound/1.

    OK, so it seems like it is not such a good idea.

    There are, however, over 200 files that contain calls to the various time
    functions that follow this pattern, eg:

    arch/arm/kernel/ecard.c:563
    if (!last || time_after(jiffies, last + 5*HZ)) {

    Perhaps they should be coverted to use a subtraction as well?

    julia



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