linux-kernel - Re: Wrong use of MAX_JIFFY

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Message-Id: <20120406120521.1a6dcd1b.akpm@linux-foundation.org>
Date:	Fri, 6 Apr 2012 12:05:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Liu Yu" <liuyums@...tor.com.cn>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: Wrong use of MAX_JIFFY_OFFSET?

On Wed, 28 Mar 2012 15:12:24 +0800
"Liu Yu" <liuyums@...tor.com.cn> wrote:

> Hi guys,
> 
> I saw a couple of places in current kernel have this kind of code:
> 
> > static inline unsigned int elapsed_jiffies_msecs(unsigned long start)
> > {
> >         unsigned long end = jiffies;
> >
> >         if (end >= start)
> >                return jiffies_to_msecs(end - start);
> >
> >         return jiffies_to_msecs(end + (MAX_JIFFY_OFFSET - start) + 1);
> > }
> 
> As you know, jiffies has a type of unsigned long, so if we know which is the
> end and
> which is the start, then (end - start) can simply figure out how much
> jiffies flies,
> without worry about the overflow.
> 
> Look at the code above, assume that there is just an overflow happening on
> jiffies: end=0 and start=~0UL.
> Since end < start, then the return value of the function is
> jiffies_to_msecs(MAX_JIFFY_OFFSET+2).
> But shouldn't the correct value be jiffies_to_msecs(1)?
> 
> could someone tell me that am I missing anything?
> 

Seems right.  The code should be

static inline unsigned long elapsed_jiffies_msecs(unsigned long start)
{
	return jiffies_to_msecs(jiffies - start);
}

Note the return type.  jiffies_to_msecs() currently returns unsigned
int.  I think it should return unsigned long.  Even then, it can still
overflow with valid inputs on HZ=100 32-bit machines.

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