linux-kernel - Re: kprobes: get rid of distinct type warning

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Message-ID: <20100104155702.GB5671@osiris.boeblingen.de.ibm.com>
Date:	Mon, 4 Jan 2010 16:57:02 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: kprobes: get rid of distinct type warning

On Wed, Dec 30, 2009 at 01:29:45PM -0800, Andrew Morton wrote:
> On Mon, 21 Dec 2009 13:02:24 +0100
> Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> 
> > Of course the patch wouldn't help for CONFIG_PREEMPT and !CONFIG_SMP since
> > we would have a comparison of a signed and and unsigned value again *sigh*.
> 
> We should fix that, shouldn't we?  Rather than working around it at one
> caller site.
> 
> : #if NR_CPUS > 1
> : #define num_online_cpus()	cpumask_weight(cpu_online_mask)
> : #define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
> : #define num_present_cpus()	cpumask_weight(cpu_present_mask)
> : #define num_active_cpus()	cpumask_weight(cpu_active_mask)
> : #define cpu_online(cpu)		cpumask_test_cpu((cpu), cpu_online_mask)
> : #define cpu_possible(cpu)	cpumask_test_cpu((cpu), cpu_possible_mask)
> : #define cpu_present(cpu)	cpumask_test_cpu((cpu), cpu_present_mask)
> : #define cpu_active(cpu)		cpumask_test_cpu((cpu), cpu_active_mask)
> : #else
> : #define num_online_cpus()	1
> : #define num_possible_cpus()	1
> : #define num_present_cpus()	1
> : #define num_active_cpus()	1
> : #define cpu_online(cpu)		((cpu) == 0)
> : #define cpu_possible(cpu)	((cpu) == 0)
> : #define cpu_present(cpu)	((cpu) == 0)
> : #define cpu_active(cpu)		((cpu) == 0)
> : #endif
> 
> The num_*() "functions" return unsigned on SMP and int on UP.  This is
> wrong.
> 
> The cpu_*() "functions" got lucky and return int in both cases.
> 
> Personally I think it's neatest if a quantity which can never be
> negative is held in an unsigned type.  Than includes anything starting
> with "num".  But for expediency's sake we could live with making these
> things consistently return "int".
> 
> Alas, changing those four num_*() "functions" to return int on SMP is a
> pretty wide-reaching change and will probably expose warts.

Looks like there are quite a lot of num_* function usages in the kernel.
Some seem to assume they return an int some assume an unsigned int.
Don't know if it's worth changing anything here.
Maybe Rusty has an opinion.
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