linux-kernel - Re: [PATCH 1/2 #tj-percpu] x86: fix build breakage on voyage

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Date:	Tue, 27 Jan 2009 14:11:08 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <htejun@...il.com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2 #tj-percpu] x86: fix build breakage on voyage


* Tejun Heo <htejun@...il.com> wrote:

> Hello, Ingo.
> 
> Ingo Molnar wrote:
> >> +#ifdef CONFIG_X86_LOCAL_APIC
> >>  	early_per_cpu_ptr(x86_cpu_to_apicid) = NULL;
> >>  	early_per_cpu_ptr(x86_bios_cpu_apicid) = NULL;
> >> +#endif
> > 
> > That patch is not acceptable - it is ugly and it adds another set of
> > #ifdefs to an already complex piece of code.
> 
> Well, although the patch itself does add #ifdef, if you look over the 
> whole series, voyager is now a much more conforming citizen in the x86 
> world.  There are several solutions to this particular one.
> 
> 1. Just let apic stuff defined and not use it in voyager if the ifdef
>    is disturbing.  IIUC, apic isn't used in voyager at all, right?
> 
> 2. Clean up early percpu stuff so that it each early percpu variable
>    doesn't need to be explicitly copied and cleared, which is the
>    actual problem here.
> 
> 3. But, then again, the current interim and ugly way of doing it isn't
>    too bad considering the small number of early per cpu users.
> 
> To me the current form doesn't look too bad but if it's too ugly, maybe 
> doing #2 is not such a bad idea such that early percpu can be 
> transferred to percpu in more systematic way.  It still feels a bit like 
> overdoing it tho.
> 
> What do you think?

This issue might be minor, but it's the death of a thousand cuts. It 
should switch to the generic x86 code, use smp_ops to wrap/express its own 
SMP weirdnesses [and extend it where needed - because _that_ is a step 
forward for the whole code - fixing build bugs isnt] and then such 
problems simply wont occur.

	Ingo
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