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
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This archive was generated by hypermail 2b29 : Wed Nov 07 2001 - 21:00:37 EST