Messages in this thread | | | Subject | Re: [PATCH] net/ipv4/*, net/core/neighbour.c jiffies cleanup | From | "Krishna Kumar" <> | Date | Thu, 8 Nov 2001 09:47:17 -0800 |
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Hi Linus,
Thanks for your clarification, it does make sense. I did only the jiffies overflowing case, and missed the case where it does not overflow.
Thanks,
- KK -----------------------------------------------------------------------------
Ok.
Let's give an example. HZ is 100, and we started just before jiffies wrapped, and we want to check that we're within one second.
So "start" equals 0xfffffff0, and "jiffies" equals 0xfffffff5.
The first if-statement will say
if (0xfffffff5 <= 0xfffffff0+100)
which is the same as
if (0xfffffff5 <= 0x54)
which is
if (0)
in short, the first statement will say that jiffies is _not_ within 100 ticks of "start", which is obviously wrong. Jiffies _is_ within 100 ticks, it is in fact just 5 ticks after "start".
The second statement will say
if (0xfffffff5 - 0xfffffff0 <= 100)
which is
if (5 <= 100)
which is
if (1)
which is _correct_. We _are_ within 100 ticks.
See?
Ok, that was wrap-around one way: the "+HZ" wrapped. Let's see the other case, which is that "jiffies" has wrapped: start is still 0xfffffff0, but jiffies has wrapped around and is 0x00000001.
The first if-statement will say
if (0x00000001 <= 0xfffffff0+100)
which is
if (0x00000001 <= 0x54)
which is
if (1)
which is correct. The second one will say
if (0x00000001 - 0xfffffff0 <= 100)
which is
if (11 <= 100)
which is
if (1)
which is correct.
In short, the _correct_ one ALWAYS gets the right answer. Even when the subtraction overflows.
While the first (and incorrect one) gets the wrong answer when the addition overflows.
Do you see the difference now?
Linus
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