Re: [RFC] kobject: add kobject_init_ng and kobject_init_and_add functions

On Fri, Nov 30, 2007 at 03:25:52PM -0500, Alan Stern wrote:
On Fri, 30 Nov 2007, Greg KH wrote:

+/**
+ * kobject_init_and_add - initialize a kobject structure and add it to the kobject hierarchy
+ * @kobj: pointer to the kobject to initialize
+ * @ktype: pointer to the ktype for this kobject.
+ * @parent: pointer to the parent of this kobject.
+ * @fmt: the name of the kobject.
+ *
+ * This function will properly initialize a kobject and then call
+ * kobject_add().
+ *
+ * If the function returns an error, the memory allocated by the kobject
+ * can be safely freed, no other functions need to be called.
+ */
+int kobject_init_and_add(struct kobject *kobj, struct kobj_type *ktype,
+ struct kobject *parent, const char *fmt, ...)
+{
+ va_list args;
+ int retval;
+
+ va_start(args, fmt);
+ retval = kobject_init_varg(kobj, ktype, parent, fmt, args);
+ va_end(args);
+ if (retval)
+ return retval;
+
+ retval = kobject_add(kobj);
+ if (retval)
+ kobject_put(kobj);

No, no!

You have recreated the problem we have been discussing during the last
couple of days. If the kobject_init_varg() routine gets an error then
the kobject will need to be deallocated manually. If the kobject_add()
routine gets an error then the cleanup invoked by kobject_put() will do
the deallocation automatically.

But the caller can't tell in which subroutine an error occurred, so it
won't know what to do when kobject_init_and_add() returns an error.

Oh crap. You're totally right. I suck.

The only way to resolve this problem is to have the _init routine
consume no resources and never fail. That way the only possible
failure mode would be if the _add routine doesn't work, in which case
either a kfree() or a kobject_put() would be acceptable.

In particular, this implies that the name should be set as part of the
_add() call, not as part of _init(). This is more in line with the way
the code tends to use kobjects anyhow. Unless people want to name
unregistered kobjects -- does this ever happen? And it if does, can
these kobjects simply be replaced by krefs?

No, the only non-registered kobjects in the tree right now are never
named. So this should be safe.

My suggestion: Have kobject_init_ng() accept a ktype pointer but not a
parent or name. Instead, make kobject_add_ng() take the parent and
name (possibly a kset also). Then when kobject_init_and_add()
encounters an error, it shouldn't do a _put() -- the caller can either
do the _put() or just do a kfree().

Why not the parent for init()? Isn't it always known at that time?
I'll dig to be sure.

Ok, second round of patches coming up...

thanks,

greg k-h
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