On Wed, Aug 8, 2012 at 5:52 PM, Paolo Bonzini <[email protected]> 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()
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.
> rcu_read_unlock()
> free()
> <dangling pointer!>
>
> If you defer the unref, you have instead
>
> VCPU thread I/O thread
> =====================================================================
> get MMIO request
> rcu_read_lock()
> walk memory map
> qdev_unmap()
> lock_devtree()
> ...
> unlock_devtree
> unref is enqueued
> ref() -> refcnt = 2
> rcu_read_unlock()
> unref() -> refcnt=1
> unref() -> refcnt = 1
>
> So this also makes patch 14 unnecessary.
>
> Paolo
>
>> +}
>
>
