Messages in this thread | | | Date | Thu, 24 Oct 2013 06:31:21 -0700 | From | Guenter Roeck <> | Subject | Re: Usage of for_each_child_of_node() |
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On 10/24/2013 12:50 AM, Thierry Reding wrote: > On Wed, Oct 23, 2013 at 09:16:44AM -0700, Guenter Roeck wrote: >> On Wed, Oct 23, 2013 at 09:10:07AM +0200, Thierry Reding wrote: >>> On Sat, Oct 12, 2013 at 10:15:03PM -0500, Rob Herring wrote: >>>> On Sat, Oct 12, 2013 at 3:54 PM, Guenter Roeck <linux@roeck-us.net> wrote: >>>>> Hi all, >>>>> >>>>> for_each_child_of_node() and similar functions increase the refcount >>>>> on each returned node and expect the caller to release the node by >>>>> calling of_node_put() when done. >>>>> >>>>> Looking through the kernel code, it appears this is hardly ever done, >>>>> if at all. Some code even calls of_node_get() on returned nodes again. >>>>> >>>>> I guess this doesn't matter in cases where devicetree is a static entity. >>>>> However, this is not (or no longer) the case with devicetree overlays, >>>>> or more generically in cases where devicetree nodes are added and >>>>> removed dynamically. >>>>> >>>>> Fundamental question: Would patches to fix this problem be accepted upstream >>>>> ? >>>> >>>> Certainly. >>>> >>>>> Or, of course, stepping a bit back: Am I missing something essential ? >>>> >>>> No. I think this is frequently wrong since it typically doesn't matter >>>> for static entries as you mention. >>> >>> Actually, I think it actually happens to be correct most of the time. >>> The reason is that for_each_child_of_node() internally calls the >>> of_get_next_child() to iterate over all children. And that function >>> already calls of_node_put() on the "previous" node. So if all the code >>> does is to iterate over all nodes to query them, then all should be >>> fine. >>> >> Good, that reduces the scope of the problem significantly. >> >>> The only case where you actually need to drop the reference on a node is >>> if you break out of the loop (so that of_get_next_child() will not be >>> called). But that's usually the case when you need to perform some >>> operation on the node, in which case it is the right thing to hold on to >>> a reference until you're done with the node. >>> >> Unfortunately, there are many cases with code such as >> >> if (error) >> return; /* or break; */ > > Well, a break isn't necessarily bad, since you could be using the node > subsequently. I imagine that depending on the exact block following the
Correct, but I meant the error case. Randomly looking through several drivers, most of them get error return handling wrong. "Winner" so far is of_regulator_match(), which doesn't release the node on error return, but does not acquire references for use afterwards either.
Something to do with my non-existing free time ;-).
Guenter
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