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 if statement the node could also be assigned to some field within a structure or similar, in which case this might still be valid. So it really needs to be evaluated on a case by case basis. If the above is actually verbatim, then yes, that's certainly an error. > or even > if (found node) > return of_node_get(node); > > in the loop. Yeah, I think all of those are probably wrong too. Thierry
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