Message-ID: <4F171E22.7000104@linux.vnet.ibm.com>
Date:	Thu, 19 Jan 2012 01:01:46 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	Alan Stern <stern@...land.harvard.edu>,
	"Luck, Tony" <tony.luck@...el.com>, Greg KH <gregkh@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Sergei Trofimovich <slyich@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	Borislav Petkov <bp@...64.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"prasad@...ux.vnet.ibm.com" <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" 
	<gouders@...bocholt.fh-gelsenkirchen.de>,
	Marcos Souza <marcos.mage@...il.com>,
	"justinmattock@...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 01/19/2012 12:20 AM, Kay Sievers wrote:

> On Wed, Jan 18, 2012 at 19:10, Alan Stern <stern@...land.harvard.edu> wrote:
> 
>> There's nothing special about the driver model code in this respect.
>> The same restriction applies wherever object lifetimes are controlled
>> by reference counting.
> 
> Right. But it might not be obvious what 's the background here:
> 
> An allocated device object(memory) usually represents an actual
> device(hardware). The object can have N users. Every of the users is
> required to take a reference to the object, which pins the object's
> memory as long as any of the N users might need to access it.
> 
> In a hotplug world, we deal with device-removal.  On disconnect, we
> usually just orphan the object, we remove it from visibility,
> disconnect the device <-> object relation.
> 
> All of the  N users with a reference can still access the memory, they
> just do not talk to a real device anymore. The invalidated/orphaned
> state is communicated otherwise by locks and flags in the device
> object. Only after all of the N users left the object alone, the
> memory of the orphan if free'd.
> 
> If in the time-window between disconnecting the object from the device
> and freeing the orphaned object's memory, the same device comes back,
> we allocate a new object which is associated with the device. It
> usually has the same name and same properties as the original one.
> 
> This way, the new object is full functional, does not conflict with
> the older one, and also all the users of the old memory are still fine
> and can cleanup a lazy as they need without much synchronization.
> 
> Now, all that might not apply to machinecheck, and it might be that
> machinecheck is fully able to handle all that just fine with the
> statically allocated same memory -- allocating new device memory on
> hotplug is still the model that should always be preferred over any
> other, if possible.
> 
> It's usually the simplest safest and most flexible for anything that
> can come and go at any time, and memory which might be in used by
> other running code.
> 


Thanks for the crystal clear explanations from both you and Alan. Now
I am convinced.. :-) And thanks to Tony for bringing this up!

Regards,
Srivatsa S. Bhat

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