Re: [PATCH] mce: fix warning messages about static struct mce_device

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 01/19/2012 12:20 AM, Kay Sievers wrote:

> On Wed, Jan 18, 2012 at 19:10, Alan Stern <stern@rowland.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