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Date: Thu, 09 Aug 2012 10:00:16 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 13/15] hotplug: introduce
 qdev_unplug_complete() to remove device from views
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To: liu ping fan <qemulist@gmail.com>
Cc: kvm@vger.kernel.org, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, Avi Kivity <avi@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Stefan Hajnoczi <stefanha@gmail.com>, =?ISO-8859-1?Q?Andreas_F=E4rber?= <afaerber@suse.de>

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.

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.

Paolo