From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH 13/15] hotplug: introduce
 qdev_unplug_complete() to remove device from views
Date: Wed, 08 Aug 2012 13:07:27 +0300
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Cc: kvm@vger.kernel.org, Jan Kiszka <jan.kiszka@siemens.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Liu Ping Fan <qemulist@gmail.com>, qemu-devel@nongnu.org,
	Blue Swirl <blauwirbel@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	=?ISO-8859-15?Q?Andreas_F=E4rber?= <afaerber@suse.de>
To: Paolo Bonzini <pbonzini@redhat.com>
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On 08/08/2012 12:52 PM, Paolo Bonzini wrote:
> Il 08/08/2012 08:25, Liu Ping Fan ha scritto:
>> +void qdev_unplug_complete(DeviceState *dev, Error **errp)
>> +{
>> +    /* isolate from mem view */
>> +    qdev_unmap(dev);
>> +    qemu_lock_devtree();
>> +    /* isolate from device tree */
>> +    qdev_unset_parent(dev);
>> +    qemu_unlock_devtree();
>> +    object_unref(OBJECT(dev));
> 
> Rather than deferring the free, you should defer the unref.  Otherwise
> the following can happen when you have "real" RCU access to the memory
> map on the read-side:
> 
>     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()
>     rcu_read_unlock()
>                                    free()
>     <dangling pointer!>

unref should follow either synchronize_rcu(), or be in a call_rcu()
callback (deferring the unref).  IMO synchronize_rcu() is sufficient
here, unplug is a truly slow path, esp. on real hardware.


-- 
error compiling committee.c: too many arguments to function

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Date: Wed, 08 Aug 2012 13:07:27 +0300
From: Avi Kivity <avi@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: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, Liu Ping Fan <qemulist@gmail.com>, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, Stefan Hajnoczi <stefanha@gmail.com>, =?ISO-8859-15?Q?Andreas_F=E4rber?= <afaerber@suse.de>

On 08/08/2012 12:52 PM, Paolo Bonzini wrote:
> Il 08/08/2012 08:25, Liu Ping Fan ha scritto:
>> +void qdev_unplug_complete(DeviceState *dev, Error **errp)
>> +{
>> +    /* isolate from mem view */
>> +    qdev_unmap(dev);
>> +    qemu_lock_devtree();
>> +    /* isolate from device tree */
>> +    qdev_unset_parent(dev);
>> +    qemu_unlock_devtree();
>> +    object_unref(OBJECT(dev));
> 
> Rather than deferring the free, you should defer the unref.  Otherwise
> the following can happen when you have "real" RCU access to the memory
> map on the read-side:
> 
>     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()
>     rcu_read_unlock()
>                                    free()
>     <dangling pointer!>

unref should follow either synchronize_rcu(), or be in a call_rcu()
callback (deferring the unref).  IMO synchronize_rcu() is sufficient
here, unplug is a truly slow path, esp. on real hardware.


-- 
error compiling committee.c: too many arguments to function