linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Message-ID: <20120312175220.GE2471@linux.vnet.ibm.com>
Date:	Mon, 12 Mar 2012 10:52:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <eag0628@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine
 call_srcu()

On Sat, Mar 10, 2012 at 11:12:29AM +0100, Peter Zijlstra wrote:
> On Thu, 2012-03-08 at 11:49 -0800, Paul E. McKenney wrote:
> > 
> > I too have used (long)(a - b) for a long time, but I saw with my own eyes
> > the glee in the compiler-writers' eyes when they discussed signed overflow
> > being undefined in the C standard.  I believe that the reasons for signed
> > overflow being undefined are long obsolete, but better safe than sorry. 
> 
> Thing is, if they break that the whole kernel comes falling down, I
> really wouldn't worry about RCU at that point. But to each their
> pet-paranoia I guess ;-)

But just because I am paranoid doesn't mean that no one is after me!  ;-)

I agree that the compiler guys would need to provide a chicken switch
due to the huge amount of code that relies on (long)(a - b) handling
overflow reasonably.  But avoiding signed integer overflow is pretty
straightforward.  For example, I use the following in RCU:

	#define UINT_CMP_GE(a, b)	(UINT_MAX / 2 >= (a) - (b))
	#define UINT_CMP_LT(a, b)	(UINT_MAX / 2 < (a) - (b))
	#define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
	#define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))

But yes, part of the reason for my doing this was to make conversations
with the usual standards-committee suspects go more smoothly.

							Thanx, Paul

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