Re: [Qemu-devel] [PATCH v3] qdev/core: fix qbus_is_full()

[Tony Krowiak]

On 1/8/19 12:06 PM, Cornelia Huck wrote:
> On Tue, 8 Jan 2019 17:50:21 +0100
> Halil Pasic <address@hidden> wrote:
>
> > On Tue, 8 Jan 2019 17:31:13 +0100
> > Cornelia Huck <address@hidden> wrote:
> >
> > > On Tue, 8 Jan 2019 11:08:56 -0500
> > > Tony Krowiak <address@hidden> wrote:
> > >    
> > > > On 12/17/18 10:57 AM, Tony Krowiak wrote:
> > > > > The qbus_is_full(BusState *bus) function (qdev_monitor.c) compares the max_index
> > > > > value of the BusState structure with the max_dev value of the BusClass structure
> > > > > to determine whether the maximum number of children has been reached for the
> > > > > bus. The problem is, the max_index field of the BusState structure does not
> > > > > necessarily reflect the number of devices that have been plugged into
> > > > > the bus.
> > > > >
> > > > > Whenever a child device is plugged into the bus, the bus's max_index value is
> > > > > assigned to the child device and then incremented. If the child is subsequently
> > > > > unplugged, the value of the max_index does not change and no longer reflects the
> > > > > number of children.
> > > > >
> > > > > When the bus's max_index value reaches the maximum number of devices
> > > > > allowed for the bus (i.e., the max_dev field in the BusClass structure),
> > > > > attempts to plug another device will be rejected claiming that the bus is
> > > > > full -- even if the bus is actually empty.
> > > > >
> > > > > To resolve the problem, a new 'num_children' field is being added to the
> > > > > BusState structure to keep track of the number of children plugged into the
> > > > > bus. It will be incremented when a child is plugged, and decremented when a
> > > > > child is unplugged.
> > > > >
> > > > > Signed-off-by: Tony Krowiak <address@hidden>
> > > > > Reviewed-by: Pierre Morel<address@hidden>
> > > > > Reviewed-by: Halil Pasic <address@hidden>
> > > > > ---
> > > > >    hw/core/qdev.c         | 3 +++
> > > > >    include/hw/qdev-core.h | 1 +
> > > > >    qdev-monitor.c         | 2 +-
> > > > >    3 files changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > Ping ...
> > > > I could not determine who the maintainer is for the three files
> > > > listed above. I checked the MAINTAINERS file and the prologue of each
> > > > individual file. Can someone please tell me who is responsible
> > > > for merging these changes? Any additional review comments?
> > > >    
> > > > > diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> > > > > index 6b3cc55b27c2..956923f33520 100644
> > > > > --- a/hw/core/qdev.c
> > > > > +++ b/hw/core/qdev.c
> > > > > @@ -59,6 +59,8 @@ static void bus_remove_child(BusState *bus, DeviceState 
> > > > > *child)
> > > > >                snprintf(name, sizeof(name), "child[%d]", kid->index);
> > > > >                QTAILQ_REMOVE(&bus->children, kid, sibling);
> > > > >    
> > > > > +            bus->num_children--;
> > > > > +
> > > > >                /* This gives back ownership of kid->child back to us.  */
> > > > >                object_property_del(OBJECT(bus), name, NULL);
> > > > >                object_unref(OBJECT(kid->child));
> > > > > @@ -73,6 +75,7 @@ static void bus_add_child(BusState *bus, DeviceState *child)
> > > > >        char name[32];
> > > > >        BusChild *kid = g_malloc0(sizeof(*kid));
> > > > >    
> > > > > +    bus->num_children++;
> > > > >        kid->index = bus->max_index++;
> > >
> > > Hm... I'm wondering what happens for insane numbers of hotplugging
> > > operations here?
> > >
> > > (Preexisting problem for busses without limit, but busses with a limit
> > > could now run into that as well.)
> > >    
> >
> > How does this patch change things? I mean bus->num_children gets
> > decremented on unplug.
>
> We don't stop anymore if max_index >= max_dev, which means that we can
> now trigger that even if max_dev != 0.

I guess I am missing your point. If max_dev == 0, then there is nothing
stopping an insane number of hot plug operations; either before this
patch, or with this patch. With the patch, once the number of children
hot plugged reaches max_dev, the qbus_is_full function will return false
and no more children can be plugged. If a child device is unplugged,
the num_children - which counts the number of children attached to the
bus - will be decremented, so it always reflects the number of children
added to the bus. Besides, checking max_index against max_dev
is erroneous, because max_index is incremented every time a child device
is plugged and is never decremented. It really operates as more of a
uinique identifier than a counter and is also used to create a unique
property name when the child device is linked to the bus as a property
(see bus_add_child function in hw/core/qdev.c).

> > Regards,
> > Halil
> >
> > > > >        kid->child = child;
> > > > >        object_ref(OBJECT(kid->child));
> > > > > diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> > > > > index a24d0dd566e3..521f0a947ead 100644
> > > > > --- a/include/hw/qdev-core.h
> > > > > +++ b/include/hw/qdev-core.h
> > > > > @@ -206,6 +206,7 @@ struct BusState {
> > > > >        HotplugHandler *hotplug_handler;
> > > > >        int max_index;
> > > > >        bool realized;
> > > > > +    int num_children;
> > > > >        QTAILQ_HEAD(ChildrenHead, BusChild) children;
> > > > >        QLIST_ENTRY(BusState) sibling;
> > > > >    };
> > > > > diff --git a/qdev-monitor.c b/qdev-monitor.c
> > > > > index 07147c63bf8b..45a8ba49644c 100644
> > > > > --- a/qdev-monitor.c
> > > > > +++ b/qdev-monitor.c
> > > > > @@ -414,7 +414,7 @@ static DeviceState *qbus_find_dev(BusState *bus, char *elem)
> > > > >    static inline bool qbus_is_full(BusState *bus)
> > > > >    {
> > > > >        BusClass *bus_class = BUS_GET_CLASS(bus);
> > > > > -    return bus_class->max_dev && bus->max_index >= bus_class->max_dev;
> > > > > +    return bus_class->max_dev && bus->num_children >= bus_class->max_dev;
> > > > >    }
> > > > >    
> > > > >    /*
> > > > >      
> > >
> > > The approach the patch takes looks sane to me.
> > >