linux-kernel - Re: [PATCH] module: add kset_obj

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Date:	Thu, 11 Apr 2013 15:53:40 +0200
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	bhelgaas@...gle.com
Subject: Re: [PATCH] module: add kset_obj_exists() and use it

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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