something about jiffies wraparound overflow

[jeff shia <tshxiayu@gmail.com>]

 Hello,

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)

#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)

But I cannot understand it has some differences comparing with the
following code.

/* code 2*/

unsigned long timeout = jiffies + HZ/2;

if(timeout < jiffies)
{
        /* not timeout...*/
}
else
{
        /* timeout processing...*/
}

questions:
  1.why the macros of time_after can solve the jiffies wraparound problem?
  2.Is there any other possibilities for the "code 2" to overflow
except the jiffies overflow?

Any help will be preferred .
Thank you,


Yours.