linux-kernel - Re: [PATCH tip/core/rcu 11/11] jiffies: Avoid undefined behavior from signed overflow

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Message-ID: <20130819004120.GU29406@linux.vnet.ibm.com>
Date:	Sun, 18 Aug 2013 17:41:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, John Stultz <john.stultz@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Kevin Easton <kevin@...rana.org>
Subject: Re: [PATCH tip/core/rcu 11/11] jiffies: Avoid undefined behavior
 from signed overflow

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@...ux.vnet.ibm.com>
> > 
> > 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@...htriplett.org>
> 

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