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Re: [Qemu-trivial] [Qemu-devel] [PATCH v2] vl.c: make current_machine as
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH v2] vl.c: make current_machine as non-global variable |
Date: |
Tue, 02 Apr 2019 18:13:07 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) |
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.