Re: [PATCH tip/core/rcu 11/11] jiffies: Avoid undefined behaviorfrom signed overflow

[Paul E. McKenney]

On Sat, Aug 17, 2013 at 08:23:51PM -0700, Josh Triplett wrote:
> On Sat, Aug 17, 2013 at 06:37:56PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > 
> > According to the C standard 3.4.3p3, overflow of a signed integer results
> > in undefined behavior.  This commit therefore changes the definitions
> > of time_after(), time_after_eq(), time_after64(), and time_after_eq64()
> > to avoid this undefined behavior.  The trick is that the subtraction
> > is done using unsigned arithmetic, which according to 6.2.5p9 cannot
> > overflow because it is defined as modulo arithmetic.  This has the added
> > (though admittedly quite small) benefit of shortening two lines of code
> > by four characters each.
> > 
> > Note that the C standard considers the cast from unsigned to
> > signed to be implementation-defined, see 6.3.1.3p3.  However, on a
> > two-complement system, an implementation that defines anything other
> > than a reinterpretation of the bits is free come to me, and I will be
> 
> s/free come/free to come/

Good catch, fixed!

							Thanx, Paul

> > happy to act as a witness for its being committed to an insane asylum.
> 
> With the typo above fixed:
> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> 

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