On Wednesday, September 14, 2016 10:28:24 AM Lukas Wunner wrote:

On Wed, Sep 14, 2016 at 03:21:27AM +0200, Rafael J. Wysocki wrote:

quoted

On Sunday, September 11, 2016 10:43:36 PM Lukas Wunner wrote:

quoted

On Sun, Sep 11, 2016 at 03:40:58PM +0200, Lukas Wunner wrote:

quoted

On Thu, Sep 08, 2016 at 11:27:45PM +0200, Rafael J. Wysocki wrote:

quoted

+/**
+ * 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.

I actually would prefer to make it impossible to link a parent to
a child at all.

Which is precisely what the code snippet above does.

All right, this means I shouldn't reply to email late in the night.  But
at least we seem to be in agreement here.

Thanks,
Rafael