netdev - Re: RFC: Use of "jiffies" vs "jiffies64" in the neighbour system.

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Message-ID: <20150325102704.GB4673@unicorn.suse.cz>
Date:	Wed, 25 Mar 2015 11:27:04 +0100
From:	Michal Kubecek <mkubecek@...e.cz>
To:	Ulf Samuelsson <ulf.samuelsson@...csson.com>
Cc:	netdev@...r.kernel.org
Subject: Re: RFC: Use of "jiffies" vs "jiffies64" in the neighbour system.

On Wed, Mar 25, 2015 at 11:11:45AM +0100, Ulf Samuelsson wrote:
> If you run HZ = 1000, then jiffies will wrap around after 49,71 days.
> This means that all time compares in the neighbour system will fail.
> 
> From what I can see from "jiffies,h" there is no attempt to detect
> the wrap-around.
> 
> #define time_after(a,b)        \
>     (typecheck(unsigned long, a) && \
>      typecheck(unsigned long, b) && \
>      ((long)(b) - (long)(a) < 0))
> #define time_before(a,b)    time_after(b,a)
> 
> #define time_after_eq(a,b)    \
>     (typecheck(unsigned long, a) && \
>      typecheck(unsigned long, b) && \
>      ((long)(a) - (long)(b) >= 0))
> #define time_before_eq(a,b)    time_after_eq(b,a)

I might have misunderstood you but those macros do handle the
wrap-around. For example, with 64-bit long and a = 1,
b = 0xffffffffffffffff, you get

  ((long)(b) - (long)(a)) = (-1) - (1) = -2 < 0

so that time_after(1, 0xffffffffffffffff) is true. It will give correct
results as long as the difference is less than half of range of the
type.

                                                         Michal Kubecek

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