netdev - Re: [PATCH 1/3] gre: used time_before for comparing jiffies

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Date:	Fri, 20 Feb 2009 16:39:35 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	yjwei@...fujitsu.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/3] gre: used time_before for comparing jiffies

David Miller <davem@...emloft.net> wrote:
>
> There is some history of this in the networking code.
> 
> If you inspect carefully, the open-coded versions of
> time comparisons in the networking have a larger window
> of acceptance.

Hmm, it looks like the macro version is actually better in this
case.

-	if (jiffies - t->err_time < IPTUNNEL_ERR_TIMEO)

This wraps at jiffies = LONG_MAX + t->err_time + 1

+	if (time_before(jiffies, t->err_time + IPTUNNEL_ERR_TIMEO))

This expands to

	(long)jiffies - (long)(t->err_time + IPTUNNEL_ERR_TIMEO) < 0

which wraps at jiffies = LONG_MAX + t->err_time + IPTUNNEL_ERR_TIMEO + 1

So assuming that IPTUNNEL_ERR_TIMEO > 0, then the macro version
wraps around after the open-coded version, which would seem to
mean that it's better, no?

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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