linux-kernel - Re: [PATCH] fix msecs_to_jiffies() to not return values greater than MAX_JIFFY

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Message-ID: <2449315.uFq6U1Y0gp@donald.sf-tec.de>
Date:	Sun, 08 May 2011 20:46:02 +0200
From:	Rolf Eike Beer <eike-kernel@...tec.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Stultz <johnstul@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] fix msecs_to_jiffies() to not return values greater than MAX_JIFFY_OFFSET

Am Dienstag, 29. M�rz 2011, 21:42:59 schrieb Thomas Gleixner:
> On Tue, 29 Mar 2011, Rolf Eike Beer wrote:
> > The documentation of msecs_to_jiffies() says:
> >  * - negative values mean 'infinite timeout' (MAX_JIFFY_OFFSET)
> >  *
> >  * - 'too large' values [that would result in larger than
> >  *   MAX_JIFFY_OFFSET values] mean 'infinite timeout' too.
> > 
> > But when you pass in e.g. MAX_JIFFY_OFFSET + 1000 for HZ = 1000 it will
> > not return MAX_JIFFY_OFFSET, but the bigger value. This makes sure that
> > the value
> 
> That's only true for 32 bit.
> 
> > returned from this function can never be bigger than MAX_JIFFY_OFFSET.
> > Also use DIV_ROUND_UP() in one place where that is open coded.
> > 
> >  unsigned long msecs_to_jiffies(const unsigned int m)
> >  {
> > 
> > +	unsigned long r;
> > 
> >  	/*
> >  	
> >  	 * Negative value, means infinite timeout:
> >  	 */
> > 
> > @@ -445,7 +446,7 @@ unsigned long msecs_to_jiffies(const unsigned int m)
> > 
> >  	 * round multiple of HZ, divide with the factor between them,
> >  	 * but round upwards:
> >  	 */
> > 
> > -	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
> > +	r = DIV_ROUND_UP(m, MSEC_PER_SEC / HZ);
> > 
> >  #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
> >  
> >  	/*
> >  	
> >  	 * HZ is larger than 1000, and HZ is a nice round multiple of
> > 
> > @@ -457,7 +458,7 @@ unsigned long msecs_to_jiffies(const unsigned int m)
> > 
> >  	if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
> >  	
> >  		return MAX_JIFFY_OFFSET;
> > 
> > -	return m * (HZ / MSEC_PER_SEC);
> > +	r = m * (HZ / MSEC_PER_SEC);
> 
> For this case the jiffies_to_msec() check should be sufficient.
> 
> >  #else
> >  
> >  	/*
> >  	
> >  	 * Generic case - multiply, round and divide. But first
> > 
> > @@ -467,9 +468,10 @@ unsigned long msecs_to_jiffies(const unsigned int
> > m)
> > 
> >  	if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
> >  	
> >  		return MAX_JIFFY_OFFSET;
> 
> Hmm, this check is silly. MUL32 is chosen, so that we cannot overflow.
> 
> > -	return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
> > +	r = (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)
> > 
>   		>> MSEC_TO_HZ_SHR32;
> >  
> >  #endif
> > 
> > +	return min_t(unsigned long, r, MAX_JIFFY_OFFSET);
> > 
> >  }
> >  EXPORT_SYMBOL(msecs_to_jiffies);
> 
> I start to wonder whether we really need these three variants or
> whether we just could go with that MUL/SHIFT based implementation and
> a final check for MAX_JIFFY_OFFSET. That would boil down to:
> 
> unsigned long msecs_to_jiffies(const unsigned int m)
> {
> 	u64 res = (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32) >> MSEC_TO_HZ_SHR32;
> 
> 	return min_t(unsigned long, (unsigned long)res, MAX_JIFFY_OFFSET);
> }
> 
> That'd avoid the whole division and msecs_to_jiffies() is not really a
> high precision function.

Ping? Is anyone going to either take my patch or do it's own reworking of that 
function?

Eike
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