linux-kernel - Re: [PATCH] timekeeping: Fix overflow in rawtime tv

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Date:	Fri, 06 Aug 2010 10:09:43 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	john stultz <johnstul@...ibm.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] timekeeping: Fix overflow in rawtime tv_nsec on 32 bit
 archs

On 08/05/2010 05:17 PM, john stultz wrote:
> On Thu, 2010-08-05 at 07:28 -0500, Jason Wessel wrote:
>> The tv_nsec is a long and when added to the shift value it can wrap
>> and become negative which later causes looping problems in the
>> getrawmonotonic().  The edge case occurs when the system has slept for
>> a short period of time of ~2 seconds.
> 
> Ah, good catch!
> 
> I reworked some of the variable names to make a little more sense and
> simplified the accumulation. Do you mind giving this a test in your
> environment that triggered the issue to make sure nothing else slipped
> in?
> 


No problem.


This looks good to me.  I even increased the delay and I can see it recovers properly.

The instrumentation shows raw_nsecs would have otherwise been negative going from 90.* to 97.* in the log.

           <...>-4801  [000]    90.105084: update_wall_time: raw_nsecs: 37283ea1
           <...>-4801  [000]    90.109078: update_wall_time: raw_nsecs: 376547b4
           <...>-4801  [000]    97.694264: update_wall_time: raw_nsecs: b1776db4
           <...>-4801  [000]    97.694270: update_wall_time: raw_nsecs: b453ffb4
           <...>-4801  [000]    97.694272: update_wall_time: raw_nsecs: 7b95c7b4

Note that I had instrumented it just after:

raw_nsecs += raw_time.tv_nsec;

We should send this over to -stable when it is considered baked because this was found in the 2.6.35 and may be a problem elsewhere as well.

Thanks,
Jason.


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