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

[liu ping fan]

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

Thanks and regards,
pingfan

> Paolo