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

[Avi Kivity <avi@redhat.com>]

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