linux-kernel - Re: [PATCH] mce: fix warning messages about static struct mce

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Date:	Wed, 18 Jan 2012 06:42:50 -0800
From:	Greg KH <gregkh@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Sergei Trofimovich <slyich@...il.com>,
	linux-kernel@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Borislav Petkov <bp@...64.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	prasad@...ux.vnet.ibm.com, Ming Lei <tom.leiming@...il.com>,
	Djalal Harouni <tixxdz@...ndz.org>,
	Borislav Petkov <borislav.petkov@....com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Andi Kleen <ak@...ux.intel.com>,
	gouders@...bocholt.fh-gelsenkirchen.de,
	Marcos Souza <marcos.mage@...il.com>, justinmattock@...il.com,
	Jeff Chua <jeff.chua.linux@...il.com>
Subject: Re: [PATCH] mce: fix warning messages about static struct mce_device

On Wed, Jan 18, 2012 at 10:31:38AM +0100, Ingo Molnar wrote:
> 
> * Greg KH <gregkh@...e.de> wrote:
> 
> > On Tue, Jan 17, 2012 at 09:38:43AM +0100, Ingo Molnar wrote:
> > > 
> > > * Greg KH <gregkh@...e.de> wrote:
> > > 
> > > > diff --git a/arch/x86/include/asm/mce.h b/arch/x86/include/asm/mce.h
> > > > index f35ce43..6aefb14 100644
> > > > --- a/arch/x86/include/asm/mce.h
> > > > +++ b/arch/x86/include/asm/mce.h
> > > > @@ -151,7 +151,7 @@ static inline void enable_p5_mce(void) {}
> > > >  
> > > >  void mce_setup(struct mce *m);
> > > >  void mce_log(struct mce *m);
> > > > -DECLARE_PER_CPU(struct device, mce_device);
> > > > +extern struct device *mce_device[CONFIG_NR_CPUS];
> > > 
> > > Minor nit, i don't think we have any other such [CONFIG_NR_CPUS] 
> > > pattern in the kernel.
> > > 
> > > This should be something like:
> > > 
> > >   DECLARE_PER_CPU(struct device *, mce_device);
> > 
> > That is what we used to have, but with just a static struct 
> > device. [...]
> 
> Which was fine in itself for a per CPU data structure - wouldnt 
> the warning be fixed by memset()-ing before registering the 
> device or such, if device registry absolutely needs a pre-zeroed 
> buffer?

It was already fixed that way, but the problem is that you can not have
statically allocated 'struct device' objects in the system.  That's what
my add-on patch fixed, also resolving the syslog messages saying there
was no release function for the device as well.

> I still think there must be some bug/assumption lurking in the 
> device layer - do you require all device allocations to be one 
> via zalloc()? Seems like a weird and unrobust requirement.

Yes, that's always been the requirement.

> I don't object to the quick fix that gets rid of the warnings, 
> but that quick fix came at the price of leaving the real bug 
> unfixed and at the price of introducing a new ugliness ;-)

Nope, all of the bugs are now fixed :)

> > [...] We really don't need this to be in the per-cpu area, a 
> > flat array should be just fine, why can't we use the 
> > CONFIG_NR_CPUS value?  Should we use something else?
> 
> By that argument we don't really need PER_CPU() areas to begin 
> with, a flat [CONFIG_NR_CPUS] array is just fine, right?

I never said that, only for this type of variable.

> Amongst other things we use PER_CPU to have an array of just 2 
> elements on a dual core system, even if it boots a 
> CONFIG_NR_CPUS=512 distro kernel. That saves RAM, and with 
> higher CONFIG_NR_CPUS values it adds up quickly.
> 
> > > Or the pointer should be attached to the CPU info structure.
> > 
> > Ok, I have no objection to that, do you want me to make a 
> > patch doing that, now that this is already in Linus's tree?
> 
> Would be nice if you could do that or some other equivalent 
> solution, i'd really not like to see the [CONFIG_NR_CPUS] 
> pattern to spread in the kernel, we spent a lot of time getting 
> rid of such uses ;-)

Ok, I'll work on resolving this.

thanks,

greg k-h
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