linux-kernel - Re: [PATCH 01/31] cpumask: Documentation

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Message-ID: <20081001091325.GA12503@elte.hu>
Date:	Wed, 1 Oct 2008 11:13:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mike Travis <travis@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/31] cpumask: Documentation


* Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Tuesday 30 September 2008 04:02:51 Mike Travis wrote:
> > +The Changes
> > +
> > +Provide new cpumask interface API.  The relevant change is basically
> > +cpumask_t becomes an opaque object.  This should result in the minimum
> > +amount of modifications while still allowing the inline cpumask functions,
> > +and the ability to declare static cpumask objects.
> > +
> > +
> > +    /* raw declaration */
> > +    struct __cpumask_data_s { DECLARE_BITMAP(bits, NR_CPUS); };
> > +
> > +    /* cpumask_map_t used for declaring static cpumask maps */
> > +    typedef struct __cpumask_data_s cpumask_map_t[1];
> > +
> > +    /* cpumask_t used for function args and return pointers */
> > +    typedef struct __cpumask_data_s *cpumask_t;
> > +    typedef const struct __cpumask_data_s *const_cpumask_t;
> > +
> > +    /* cpumask_var_t used for local variable, definition follows */
> > +    typedef struct __cpumask_data_s	cpumask_var_t[1]; /* SMALL NR_CPUS */
> > +    typedef struct __cpumask_data_s	*cpumask_var_t;	  /* LARGE NR_CPUS */
> > +
> > +    /* replaces cpumask_t dst = (cpumask_t)src */
> > +    void cpus_copy(cpumask_t dst, const cpumask_t src);
> 
> Hi Mike,
> 
>     I have several problems with this patch series.  First, it's a flag day 
> change, which means it isn't bisectable and can't go through linux-next.  
> Secondly, we still can't hide the definition of the cpumask struct as long as 
> they're passed as cpumask_t, so it's going to be hard to find assignments 
> (illegal once we allocate nr_cpu_ids bits rather than NR_CPUS), and on-stack 
> users.
> 
>     Finally, we end up with code which is slightly more opaque than the 
> current code, with two new typedefs.  And that's an ongoing problem.
> 
>     I took a slightly divergent line with my patch series, and introduced a 
> parallel cpumask system which always passes and returns masks by pointer:
> 
> 	cpumask_t -> struct cpumask
> 	on-stack cpumask_t -> cpumask_var_t (same as your patch)
> 	cpus_and(dst, src1, src2) etc -> cpumask_and(&dst, &src1, &src2)
> 	cpumask_t cpumask_of_cpu(cpu) -> const struct cpumask *cpumask_of(cpu)
> 	cpumask_t cpu_online_map etc -> const struct cpumask *cpu_online_mask etc.
> 
> The old ops are expressed in terms of the new ops, and can be phased out over 
> time.

that looks very sane to me.

one small request:

> I'll commit these to my quilt series today.

IMHO, an infrastructure change of this magnitude should absolutely be 
done via the Git space. This needs a ton of testing and needs bisection, 
a real Git track record, etc.

	Ingo
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