Re: [PATCH] module: add kset_obj_exists() and use it

[Veaceslav Falico]

On Thu, Apr 11, 2013 at 06:28:31AM -0700, Greg KH wrote:
> On Thu, Apr 11, 2013 at 11:55:37AM +0200, Veaceslav Falico wrote:
> > However, I think my patch still adds something good, cause now we have 2
> > cases where we basically do:
> >
> > k = kset_find_obj();
> > if (!k)
> > 	return;
> > kobject_put(k);
> >
> > which adds useless overhead (by using kobject_get()/kobject_put(), and
> > kobject_release() - which is called from kobject_put()) - where we should
> > only verify if there exists a kobject with the specified name.
> >
> > Should I resend it with a properly fixed commit message, or it's really not
> > needed?
>
> I don't think it's really needed, there is no speed/overhead issue here
> and you need to do the kobject_get/put stuff anyway if you are trying to
> look at a kobject.

This is the point, actually, that we don't need to look at a kobject. We
only need to know if it existed that time or not, here are those two
examples of code:

static int mod_sysfs_init(struct module *mod)
{
        int err;
        struct kobject *kobj;

	...

        kobj = kset_find_obj(module_kset, mod->name);
        if (kobj) {
                printk(KERN_ERR "%s: module is already loaded\n", mod->name);
                kobject_put(kobj);
                err = -EINVAL;
                goto out;
        }

	...

So we just verify if there's a kobject with mod->name, and if it exists -
_put() it back and return, otherwise do nothing (with it).

Same here:

static char *make_slot_name(const char *name)
{
	...

        for (;;) {
                struct kobject *dup_slot;
                dup_slot = kset_find_obj(pci_slots_kset, new_name);
                if (!dup_slot)
                        break;
                kobject_put(dup_slot);

	...

We look if there exists a kobject named new_name in pci_slots_kset, if yes
- free it and try another name, if not - then we're good to go.

In both examples we don't look at that kobject, and only uselessly
_get()/_put() it. And it looks a bit ugly. After the patch, in both cases,
it takes only one call to kset_obj_exists() to find out if the object
exists at that time.

However, I have absolutely no knowledge/experience in this domain and might
for sure be missing something. Sorry if it's the case.

> thanks,
>
> greg k-h
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