Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-08-27 22:53, Avi Kivity wrote:
> On 08/27/2012 12:38 PM, Jan Kiszka wrote:
>>>> Even worse, apply
>>>> restrictions on how the dispatched objects, the regions, have to be
>>>> treated because of this.
>>>
>>> Please elaborate.
>>
>> The fact that you can't manipulate a memory region object arbitrarily
>> after removing it from the mapping because you track the references to
>> the object that the region points to, not the region itself. The region
>> remains in use by the dispatching layer and potentially the called
>> device, even after deregistration.
> 
> That object will be a container_of() the region, usually literally but
> sometimes only in spirit.  Reference counting the regions means they
> cannot be embedded into other objects any more.

I cannot follow the logic of the last sentence. Reference counting just
means that we track if there are users left, not necessarily that we
perform asynchronous releases. We can simply wait for those users to
complete.

> 
> We can probably figure out a way to flush out accesses.  After switching
> to rcu, for example, all we need is synchronize_rcu() in a
> non-deadlocking place.  But my bet is that it will not be needed.

If you properly flush out accesses, you don't need to track at device
level anymore - and mess with abstraction layers. That's my whole point.

Jan



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