linux-kernel - Re: [PATCH RFC tip/core/rcu 13/20] rcu: increase synchronize_sched

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Message-ID: <20101220134051.GA29577@Krystal>
Date:	Mon, 20 Dec 2010 08:40:51 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	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
Subject: Re: [PATCH RFC tip/core/rcu 13/20] rcu: increase
	synchronize_sched_expedited() batching

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Sun, 2010-12-19 at 08:35 -0800, Paul E. McKenney wrote:
> > >  (int)((unsigned)(a) - (unsigned)(b)) < 0
> > 
> > Unfortunately, no.  :-(
> > 
> > The (int) converts from unsigned to signed, and if the upper bit of
> > the unsigned difference is non-zero, then the paragraph I quoted above
> > applies, and the standard allows the compiler to do whatever it wants.
> > 
> As noted in the previous reply, that would render quite a lot of our
> time-keeping code broken. I think its safe to assume this works.
> 
> Look at time_after() for example:
> 
> #define time_after(a,b)		\
> 	(typecheck(unsigned long, a) && \
> 	 typecheck(unsigned long, b) && \
> 	 ((long)(b) - (long)(a) < 0))

I agree with Peter: as long as the difference value is expected not to
overflow a signed long, the time_after() approach should be safe.

Now it depends if the usage Paul spotted is expected to have a
difference that overflows a signed int. It's not clear to me that it's
realistically possible from reading the patch, but I might be missing
something.

And by the way, if the difference is expected to overflow a signed int,
then we're only a factor two away from overflowing an unsigned int, so
the whole approach would be fragile.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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