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

[Avi Kivity <avi@redhat.com>]

On 08/27/2012 06:01 PM, Jan Kiszka wrote:
> 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.

I don't see how.  Suppose you add a reference count to MemoryRegion. 
How do you delay its containing object's destructor from running?  Do
you iterate over all member MemoryRegion and examine their reference counts?

Usually a reference count controls the lifetime of the reference counted
object, what are you suggesting here?

> > 
> > 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.

To flush out an access you need either rwlock_write_lock() or
synchronize_rcu() (depending on the implementation).  But neither of
these can be run from an rcu read-side critical section or
rwlock_read_lock().

You could defer the change to a bottom half, but if the hardware demands
that the change be complete before returning, that doesn't work.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.