Re: [PATCH] net/ipv4/*, net/core/neighbour.c jiffies cleanup

[Linus Torvalds <torvalds@xxxxxxxxxxxxx>]

On Wed, 7 Nov 2001, Andreas Dilger wrote:
>
> No, only a limited number of them cast to a signed value, which means
> that a large number of them get the comparison wrong in the case of
> jiffies wrap (where the difference is a large unsigned value, and not
> a small negative number).

No.

Unsigned arithmetic is fine. The _correct_ way to test whether something
is in within

        [ start , start+HZ ]

is to do

        if (jiffies - start <= HZ)

try it. The C language guarantees that unsigned arithmetic works in a
"modulo power of two" fashion, which means that it _is_ ok to do
arithmetic on unsigned longs, and jiffy wrapping does not matter. No need
to cast to "signed" or anything else.

In short: It is wrong to do

        if (jiffies <= start+HZ)

and it is _right_ to do

        if (jiffies - start <= HZ)

(as long as "start" is "unsigned long" like jiffies).

                Linus