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

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Date:	Thu, 8 Mar 2012 11:49:05 -0800
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 Tue, Mar 06, 2012 at 04:38:18PM +0100, Peter Zijlstra wrote:
> On Tue, 2012-03-06 at 23:17 +0800, Lai Jiangshan wrote:
> > On Tue, Mar 6, 2012 at 6:58 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Tue, 2012-03-06 at 17:57 +0800, Lai Jiangshan wrote:
> > >>  /*
> > >> + * 'return left < right;' but handle the overflow issues.
> > >> + * The same as 'return (long)(right - left) > 0;' but it cares more.
> > >
> > > About what? And why? We do the (long)(a - b) thing all over the kernel,
> > > why would you care more?
> > 
> > @left is constants of  the callers(callbacks's snapshot), @right
> > increases very slow.
> > if (long)(right - left) is a big negative, we have to wait for a long
> > time in this kinds of overflow.
> > this kinds of overflow can not happen in this safe_less_than()
> 
> I'm afraid I'm being particularly dense, but what?!

I have been converting the "(long)(a - b)" stuff in RCU to use unsigned
arithmetic.  The ULONG_CMP_GE() and friends in rcupdate.h are for this
purpose.

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.

							Thanx, Paul

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