On Sun, Sep 11, 2016 at 03:40:58PM +0200, Lukas Wunner wrote:
> On Thu, Sep 08, 2016 at 11:27:45PM +0200, Rafael J. Wysocki wrote:
> > +/**
> > + * device_is_dependent - Check if one device depends on another one
> > + * @dev: Device to check dependencies for.
> > + * @target: Device to check against.
> > + *
> > + * Check if @dev or any device dependent on it (its child or its consumer
> > etc)
> > + * depends on @target. Return 1 if that is the case or 0 otherwise.
> > + */
> > +static int device_is_dependent(struct device *dev, void *target)
> > +{
> > + struct device_link *link;
> > + int ret;
> > +
> > + ret = device_for_each_child(dev, target, device_is_dependent);
> > + list_for_each_entry(link, &dev->links_to_consumers, s_node) {
> > + if (WARN_ON(link->consumer == target))
> > + return 1;
> > +
> > + ret = ret || device_is_dependent(link->consumer, target);
> > + }
> > + return ret;
> > +}
>
> What happens if someone tries to add a device link from a parent
> (as the consumer) to a child (as a supplier)? You're only checking
> if target is a consumer of dev, for full correctness you'd also have
> to check if target is a parent of dev. (Or grandparent, or great-
> grandparent, ... you need to walk the tree up to the root.)
>
>
> The function can be sped up by returning immediately if a match
> is found instead of continuing searching and accumulating the
> result in ret, i.e.:
>
> if (device_for_each_child(dev, target, device_is_dependent))
> return 1;
>
> and in the list_for_each_entry block:
>
> if (device_is_dependent(link->consumer, target))
> return 1;
>
> Then at the end of the function "return 0".
>
>
> I'd move the WARN_ON() to the single invocation of this function in
> device_link_add(), that way it's possible to use the function as a
> helper elsewhere should the need arise.
Oh I'm grasping only now, you want to emit a WARN for *every*
infringing child/consumer. That could lead to a WARN flood if
a developer accidentally does something really dumb, like linking
the PCI root to some PCI endpoint device, but fair enough.
The point about linking a parent to a child still stands however.
I think a simple way to check this is to just add
if (WARN_ON(dev == target))
return 1;
at the top of the function, because when someone tries to link
a parent to a child, when recursing from the parent downward
one will eventually hit that child. This will also prevent
someone from linking a device to itself.
Best regards,
Lukas
