Re: [Qemu-devel] [PATCH v3 17/19] target-i386: fix apic object leak when CPU is deleted

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 13 Jul 2016 17:26:18 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> On Wed, 13 Jul 2016 12:04:44 -0300
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > On Wed, Jul 06, 2016 at 08:20:53AM +0200, Igor Mammedov wrote:  
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > >  target-i386/cpu.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > index 04c0b79..2fa445d 100644
> > > --- a/target-i386/cpu.c
> > > +++ b/target-i386/cpu.c
> > > @@ -2765,6 +2765,7 @@ static void x86_cpu_apic_create(X86CPU *cpu, Error **errp)
> > >  
> > >      object_property_add_child(OBJECT(cpu), "lapic",
> > >                                OBJECT(cpu->apic_state), &error_abort);
> > > +    object_unref(OBJECT(cpu->apic_state));    
> > 
> > What kind of event can trigger object_unparent() or
> > object_del_property() on "lapic"? Can we guarantee that the child
> > property will never be deleted by any other code, only by
> > x86_cpu_unrealizefn() and object_finalize(cpu)?  
> code path that triggers unparent of lapic implicitly is
> cpu instance removal when it deletes all children.
> 
> So unless someone adds explicit lapic removal somewhere in target-i386/cpu.c
Well, I've wrote nonsense here as I do remove child explicitly
in x86_cpu_unrealizefn(), so it's fine to set cpu->apic_state to NULL
as you suggest in 18/19.

The other way around might be call only apic_state->unrealize() explicitly
from x86_cpu_unrealizefn() and let QOM do unparenting/finalizing
automatically for us. I'd even prefer this one over the former.
Which one would you prefer?

> I don't see how it could be deleted by other code path.
That point still stands.

> 
> > Because with this change, deleting the property will leave us
> > with with a dangling cpu->apic_state pointer.  
> since there aren't other place that deletes lapic property we won't get it
> dangling pointer, see the next patch comment for call chain
> 
> >   
> > >  
> > >      qdev_prop_set_uint8(cpu->apic_state, "id", cpu->apic_id);
> > >      /* TODO: convert to link<> */
> > > -- 
> > > 2.7.0
> > >     
> >   
> 
>