netdev - Wrong use of MAX_JIFFY

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Date:	Tue, 27 Mar 2012 09:25:16 +0800
From:	"Liu Yu" <liuyums@...tor.com.cn>
To:	<netdev@...r.kernel.org>
Subject: Wrong use of MAX_JIFFY_OFFSET?

Hi guys,

I saw a couple of places in current kernel have this kind of code:

> static inline unsigned int elapsed_jiffies_msecs(unsigned long start)
> {
>         unsigned long end = jiffies;
>
>         if (end >= start)
>                return jiffies_to_msecs(end - start);
>
>         return jiffies_to_msecs(end + (MAX_JIFFY_OFFSET - start) + 1);
> }

As you know, jiffies has a type of unsigned long, so if we know which is the
end and
which is the start, then (end - start) can simply figure out how much
jiffies flies,
without worry about the overflow.

Look at the code above, assume that there is just an overflow happening on
jiffies: end=0 and start=~0UL.
Since end < start, then the return value of the function is
jiffies_to_msecs(MAX_JIFFY_OFFSET+2).
But shouldn't the correct value be jiffies_to_msecs(1)?

could someone tell me that am I missing anything?

Thanks,
Yu

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