Re: [Qemu-devel] [PATCH 13/15] hotplug: introduce qdev_unplug_complete() to remove device from views

[Marcelo Tosatti]

On Fri, Aug 10, 2012 at 02:42:58PM +0800, liu ping fan wrote:
> On Thu, Aug 9, 2012 at 4:00 PM, Paolo Bonzini <address@hidden> wrote:
> > Il 09/08/2012 09:28, liu ping fan ha scritto:
> >>> >     VCPU thread                    I/O thread
> >>> > =====================================================================
> >>> >     get MMIO request
> >>> >     rcu_read_lock()
> >>> >     walk memory map
> >>> >                                    qdev_unmap()
> >>> >                                    lock_devtree()
> >>> >                                    ...
> >>> >                                    unlock_devtree
> >>> >                                    unref dev -> refcnt=0, free enqueued
> >>> >     ref()
> >> No ref() for dev here, while we have ref to flatview+radix in my patches.
> >> I use rcu to protect radix+flatview+mr refered. As to dev, its ref has
> >> inc when it is added into mem view -- that is
> >> memory_region_add_subregion -> memory_region_get() {
> >> if(atomic_add_and_return()) dev->ref++  }.
> >> So not until reclaimer of mem view, the dev's ref is hold by mem view.
> >> In a short word, rcu protect mem view, while device is protected by refcnt.
> >
> > But the RCU critical section should not include the whole processing of
> > MMIO, only the walk of the memory map.
> >
> Yes, you are right.  And I think cur_map_get() can be broken into the
> style "lock,  ref++, phys_page_find(); unlock".  easily.
> 
> > And in general I think this is a bit too tricky... I understand not
> > adding refcounting to all of bottom halves, timers, etc., but if you are
> > using a device you should have explicit ref/unref pairs.
> >
> Actually, there are pairs -- when dev enter mem view, inc ref; and
> when it leave, dec ref.
> But as Avi has pointed out, the mr->refcnt introduce complicate and no
> gain. So I will discard this design

The plan that you refer that has been relatively well thought out, IIRC. 
Instead of designing something, i would try to understand/improve on
that.

> Thanks and regards,
> pingfan
> 
> > Paolo
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