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As you mentioned the code for comparison: > /* code 2*/ > unsigned long timeout = jiffies + HZ/2; the code has no problem with jiffies wrapping around as long as the values are compared in a right way. For a 32 bit platform the counter wraps around only once every 50 day when the value of HZ 1000. so if your code is prepared to face this event it will work fine. > 2.Is there any other possibilities for the "code 2" to overflow > except the jiffies overflow? No. The better option would be to use the inline macros: > #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. > 1.why the macros of time_after can solve the jiffies > wraparound problem? As it is clear that time_after evaluates true, when a, as a snapshot of jiffies, represents a time after b, These inlines deals with the timer wrapping correctly, because if the timer wrap changes in the future , you won't have to alter the driver code. Typechecks are performed at the compiled time, that variables are of the same type. the code works by first converting the values to unsigned long, subtracting them and then comparing the result. so it is the safe way and most encoraged.. If you need to know the difference between two instances of jiffies in a safe way, you can use the same trick: diff = (long)t2 - (long)t1;. regards lk. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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