Re: [Qemu-devel] [PATCH v2] vl.c: make current_machine as non-global variable

[Like Xu <like.xu@linux.intel.com>]

On 2019/4/3 0:13, Markus Armbruster wrote:
> Igor Mammedov <imammedo@redhat.com> writes:
> 
>> On Tue, 2 Apr 2019 21:09:39 +0800
>> Like Xu <like.xu@linux.intel.com> wrote:
>>
>>> On 2019/4/2 19:27, Markus Armbruster wrote:
>>>> Like Xu <like.xu@linux.intel.com> writes:
>>>>    
>>>>> This patch makes the remaining dozen or so uses of the global
>>>>> current_machine outside vl.c use qdev_get_machine() instead,
>>>>> and then make current_machine local to vl.c instead of global.
>>>>>
>>>>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>>>>
>>>> You effectively replace
>>>>
>>>>       current_machine
>>>>
>>>> by
>>>>
>>>>       MACHINE(qdev_get_machine())
>>>>
>>>> qdev_get_machine() uses container_get(), which has a side effect: any
>>>> path component that doesn't exist already gets created as "container"
>>>> object.  In case of qdev_get_machine(), that's just "/machine".
>>>>
>>>> Creating "/machine" as "container" is of course wrong.  You therefore
>>>> must not use qdev_get_machine() before main() creates "/machine".  It
>>>> does like this:
>>>>
>>>>       object_property_add_child(object_get_root(), "machine",
>>>>                                 OBJECT(current_machine), &error_abort);
>>>>
>>>> I recently had several cases of code rearrangements explode because the
>>>> reordered code called qdev_get_machine() too early.  Makes me rather
>>>> skeptical about this patch.  To be frank, I consider qdev_get_machine()
>>>> a trap for the unwary.  container_get(), too.
>>>>
>>>> If we decide using it to make current_machine static a good idea anyway,
>>>> we need to check the new uses carefully to make sure they can't run
>>>> before main() creates "/machine".
>>
>> maybe we can assert in qdev_get_machine() if machine hasn't been created yet?
>> with this at least it will be hard to misuse function or catch invalid users.
>> (but it still might miss some use cases/CLI options which are not tested)
> 
> Good idea.  When my code created "/machine" as a container, debugging
> the resulting crash took me a bit of time.  The assertion you propose
> would've saved me some.
> 
> 

Good idea and please help review if this assertion in qdev_get_machine() 
could help for your code rearrangement:

       if (dev == NULL) {
           dev = container_get(object_get_root(), "/machine");
       }

+     assert(object_dynamic_cast(dev, TYPE_MACHINE) != NULL);
       return dev;