On Thu, Oct 24, 2013 at 06:31:21AM -0700, Guenter Roeck wrote: > 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@xxxxxxxxxxxx> 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 ;-). Well, that's better than boring, isn't it? =) Thierry
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