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

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Date:	Tue, 6 Mar 2012 23:17:41 +0800
From:	Lai Jiangshan <eag0628@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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()

>
>> + */
>> +static inline
>> +bool safe_less_than(unsigned long left, unsigned long right, unsigned long max)
>> +{
>> +       unsigned long a = right - left;
>> +       unsigned long b = max - left;
>> +
>> +       return !!a && (a <= b);
>> +}
> --
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