linux-kernel - Re: [RFC PATCH] time: Fix truncation in jiffies_to

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Message-ID: <20140408141543.2de9449d@gandalf.local.home>
Date:	Tue, 8 Apr 2014 14:15:43 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Tony Luck <tony.luck@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Mauro Chehab <m.chehab@...sung.com>
Subject: Re: [RFC PATCH] time: Fix truncation in jiffies_to_usecs()

On Tue, 8 Apr 2014 19:49:51 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Mon, Apr 07, 2014 at 10:34:51PM -0700, Tony Luck wrote:
> > On Mon, Apr 7, 2014 at 3:25 PM, Tony Luck <tony.luck@...el.com> wrote:
> > 
> > > c) If not this ... then what? Separate routine to convert large numbers
> > >    of jiffies to usec/nsecs?  Should we make the existing one barf when
> > >    handed a number that overflows?
> > 
> > Having thought about this a bit more - I'm leaning towards leaving
> > jiffies_to_usecs() alone, but using it as a model for a from-scratch
> > implementation of:
> > u64 jiffies_to_nsecs(const unsigned long j)
> > {
> > }
> > 
> > This is what the uptime tracer actually needs - and there is only
> > one user of jiffies_to_nsecs() to worry about.
> 
> I'm not sure I get what you're trying to do. We already have jiffies_to_nsecs().
> Anyway I'll just wait and check out the next patch :)

I believe the issue is the way it's implemented:

static inline u64 jiffies_to_nsecs(const unsigned long j)
{
	return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC;
}

The problem is with jiffies_to_usecs(). Which we probably should
change.

With HZ = 100,
1 second jiffies_to_usecs(100) = 1000,000.
1 minute jiffies_to_usec(6000) = 60,000,000.
1 hour jiffies_to_usecs(360000) = 3,600,000,000
1 hour 11 minutes 35 seconds -
    jiffies_to_usecs(429500) = 4,295,000,000

2^32 = 4294967296  <  4,295,000,000

Overflow!

That means after 1 hour, 11 minutes and 35 seconds, jiffies_to_usecs()
will return a reset number. Time will go backwards. It doesn't matter
what you typecast the return value of jiffies_to_usecs() to, the result
is wrong.

Actually, I like Tony's first patch. I really think jiffies_to_usecs()
should return a u64 number.

-- Steve
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