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

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Message-ID: <1292841231.2708.136.camel@laptop>
Date:	Mon, 20 Dec 2010 11:33:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, 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
Subject: Re: [PATCH RFC tip/core/rcu 13/20] rcu: increase
 synchronize_sched_expedited() batching

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))

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