Re: [Qemu-devel] [PATCH v11 01/12] qmp: delete qemu opts when delete an object

[Yang Hongyang]

On 09/24/2015 05:42 PM, Markus Armbruster wrote:
> Yang Hongyang <address@hidden> writes:
>
> > On 09/24/2015 03:43 PM, Markus Armbruster wrote:
> > > This has finally reached the front of my review queue.  I apologize for
> > > the loooong delay.
> > >
> > > Copying Paolo for another pair of eyeballs (he wrote this code).
> > [...]
> > > > +
> > > > +    opts = qemu_opts_find(qemu_find_opts_err("object", NULL), id);
> > > > +    qemu_opts_del(opts);
> > >
> > > qemu_find_opts_err("object", &error_abort) please, because when it
> > > fails, we want to die right away, not when the null pointer it returns
> > > gets dereferenced.
> >
> > Thanks for the review.
> > Jason, do you want me to propose a fix on top of this series or simply drop
> > this for now because this patch is an independent bug fix and won't affect the
> > other filter patch series.
> >
> > > Same sloppiness in netdev_del_completion() and qmp_netdev_del(), not
> > > your patch's fault.
> > >
> > > Elsewhere, we store the QemuOpts in the object just so we can delete it:
> > > DeviceState, DriveInfo.  Paolo, what do you think?
> >
> > I don't get it. Currently, only objects created at the beginning through
> > QEMU command line will be stored in the QemuOpts, objects that created
> > with object_add won't stored in QemuOpts. Do you mean for DeviceState,
> > DriveInfo they store there QemuOpts explicity so that they can delete it?
> > Why don't we just delete it from objects directly instead?
>
> Let me elaborate.

Thanks very much for the elaboration.

> We have the same pattern in multiple places: some kind of object gets
> configured via QemuOpts, and an object's QemuOpts need to stay around
> until the object dies.
>
> Example 1: Block device backends
>
>      DriveInfo has a member opts.
>
>      drive_new() stores the QemuOpts in dinfo->opts.
>
>      drive_info_del() destroys dinfo->opts.
>
>      Note: DriveInfo member opts is always non-null.  But not every
>      BlockBackend has a DriveInfo.
>
> Example 2: Device frontends
>
>      DeviceState has a member opts.
>
>      qdev_device_add() stores the QemuOpts in dev->opts.
>
>      device_finalize() destroys dev->opts.
>
>      Note: DeviceState member opts may be null (not every device is
>      created by qdev_device_add()).  Fine, because qemu_opts_del(NULL) is
>      a no-op.
>
> Example 3: Character device backends
>
>      CharDriverState has a member opts.
>
>      qemu_chr_new_from_opts() stores the QemuOpts in chr->opts.
>
>      qemu_chr_delete() destroys chr->opts.

1-3 store there ops in there own state, not in global ops group right?

> Example 4: Network device backends
>
>      Two cases
>
>      A. netdev
>
>         qmp_netdev_add() does not store the QemuOpts.
>
>         qmp_netdev_del() still needs to destroy it.  It has to find it
>         somehow.  Here's how it does it:
>
>             opts = qemu_opts_find(qemu_find_opts_err("netdev", NULL), id);
>             if (!opts) {
>                 error_setg(errp, "Device '%s' is not a netdev", id);
>                 return;
>             }
>
>         The !opts condition is a non-obvious way to test "not created
>         with -netdev", see commit 645c949.  Note that the commit's claim
>         that qemu_opts_del(NULL) crashes is no longer true since commit
>         4782183.
>
>      B. Legacy net
>
>         hmp_host_net_add() does not store the QemuOpts.

I'm afraid it does store the QemuOpts, but not in it's own state.
net/net.c:
1088     qemu_opt_set(opts, "type", device, &error_abort);
This will store the QemuOpts, or am I misunderstood it?

>         hmp_host_net_remove() still needs to destroy it.  I can't see
>         where that happens, and I'm not sure it does.
>
> Example 5: Generic object
>
>      object_create() does not store the QemuOpts.
>
>      It still needs to be destroyed along with the object.  It isn't, and
>      your patch fixes it.
>
> Personally, I find the technique in example 1-3 easier to understand
> than the one in example 4-5.

I agree that opts should not be used to determine not created something
while there's case when something created but Opts not stored.

> .

--
Thanks,
Yang.