netdev - RE: Wrong use of MAX_JIFFY

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B6EBA@saturn3.aculab.com>
Date:	Tue, 27 Mar 2012 09:54:55 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Liu Yu" <liuyums@...tor.com.cn>, <netdev@...r.kernel.org>
Subject: RE: Wrong use of MAX_JIFFY_OFFSET?

 

> -----Original Message-----
> From: netdev-owner@...r.kernel.org 
> [mailto:netdev-owner@...r.kernel.org] On Behalf Of Liu Yu
> Sent: 27 March 2012 02:25
> To: netdev@...r.kernel.org
> Subject: Wrong use of MAX_JIFFY_OFFSET?
> 
> 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?

That code has confused me...
MAX_JIFFIES_OFFSET appears to have something to do with
timespec_to_jiffies() rather than being anything to so with
the way 'jiffies' itself wraps.

Since arithmetic wrap is well defined for unsigned values
the correct code should just be:
	return jiffies_to_ms(jiffies - start);
The code above is completly fubar when jiffies wraps.

	David


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