linux-kernel - Re: gettimeofday() jumping into the future

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Message-ID: <47F4D147.7060605@superbug.co.uk>
Date:	Thu, 03 Apr 2008 13:44:55 +0100
From:	James Courtier-Dutton <James@...erbug.co.uk>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Tim Ricketts <tr@...th.li>, Michael Smith <msmith@...h.org>,
	linux-kernel@...r.kernel.org, Andy Wingo <wingo@...endo.com>,
	tglx@...utronix.de
Subject: Re: gettimeofday() jumping into the future

James Courtier-Dutton wrote:
> James Courtier-Dutton wrote:
>> than half of the max value cycle_now can have, one can take a very
>> simple approach.
>> Treat the cycle_now, cycle_last value as signed.
>> If they have same sign, do the comparison as unsigned.
>> If they have different signs, do the comparision as signed.
>>
>> James
>>
> Sorry,
> >Treat the cycle_now, cycle_last value as signed.
> >If they have same sign, do the comparison as unsigned.
> >If they have different signs, do the comparision as signed.
> is wrong.
>
It should be:
treat cycle_now and cycle_last as unsigned.
unsigned tmp;
tmp = cycle_now - cycle_last;
if (tmp > max_difference_threshold) cycle_now = cycle_last;

This correctly handles cycle_now going backwards as well as wrap around.
The only way to catch all edge cases is to be able to make an assumption
on the maximum acceptable difference between cycle_now and cycle_last,
where difference is the shortest distance between values if they were
pointers into a ring buffer. The other assumption made here is that the 
wrap around only happens at max_uint of cycle_now.


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