Re: something about jiffies wraparound overflow

[Bodo Eggert <harvested.in.lkml@7eggert.dyndns.org>]

jeff shia <tshxiayu@gmail.com> wrote:

> we use the following to solve the problem of jiffies wraparound.
> #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)

[...]

> But I cannot understand it has some differences comparing with the
> following code.
> 
> /* code 2*/
> 
> unsigned long timeout = jiffies + HZ/2;
> 
> if(timeout < jiffies)

> questions:
>   1.why the macros of time_after can solve the jiffies wraparound problem?

Because the overflows get compensated. It's a property of Galois Fields.

>   2.Is there any other possibilities for the "code 2" to overflow
> except the jiffies overflow?

timeout might overflow, too.
-- 
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