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

[Like Xu]

On 2019/4/3 0:13, Markus Armbruster wrote:
> Igor Mammedov <address@hidden> writes:
>
> > On Tue, 2 Apr 2019 21:09:39 +0800
> > Like Xu <address@hidden> wrote:
> >
> > > On 2019/4/2 19:27, Markus Armbruster wrote:
> > > > Like Xu <address@hidden> 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 <address@hidden>
> > > >
> > > > 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;