From: Greg KH <gregkh@linux.com>
Subject: Re: [PATCH 1/2] kobject: introduce kobj_completion
Date: Tue, 10 Sep 2013 11:06:27 -0700
Message-ID: <20130910180627.GA4274@kroah.com>
References: <522F5A6A.2000201@suse.com>
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Cc: ext4 development <linux-ext4@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>
To: Jeff Mahoney <jeffm@suse.com>
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In-Reply-To: <522F5A6A.2000201@suse.com>
Sender: linux-ext4-owner@vger.kernel.org

On Tue, Sep 10, 2013 at 01:44:10PM -0400, Jeff Mahoney wrote:
> ext4 exports per-filesystem information via sysfs. The lifetime rules
> have historically been painful for this but the solution has been to pair
> the kobject with a completion and call complete in the kobject's
> release function.
> 
> Since this is a pattern I've used in btrfs as well, it makes sense to
> turn the pairing into a convenience structure with a standard API.
> 
> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---
>  include/linux/kobj_completion.h |   18 +++++++++++++++
>  lib/kobject.c                   |   47 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
> 
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ b/include/linux/kobj_completion.h	2013-09-10 12:58:03.530554144 -0400
> @@ -0,0 +1,18 @@
> +#ifndef _KOBJ_COMPLETION_H_
> +#define _KOBJ_COMPLETION_H_
> +
> +#include <linux/kobject.h>
> +#include <linux/completion.h>
> +
> +struct kobj_completion {
> +	struct kobject kc_kobj;
> +	struct completion kc_unregister;
> +};
> +
> +#define kobj_to_kobj_completion(kobj) \
> +	container_of(kobj, struct kobj_completion, kc_kobj)
> +
> +void kobj_completion_init(struct kobj_completion *kc, struct kobj_type *ktype);
> +void kobj_completion_release(struct kobject *kobj);
> +void kobj_completion_del_and_wait(struct kobj_completion *kc);
> +#endif /* _KOBJ_COMPLETION_H_ */
> --- a/lib/kobject.c	2013-09-10 12:57:54.198666613 -0400
> +++ b/lib/kobject.c	2013-09-10 13:16:31.750607946 -0400
> @@ -13,6 +13,7 @@
>   */
>  
>  #include <linux/kobject.h>
> +#include <linux/kobj_completion.h>
>  #include <linux/string.h>
>  #include <linux/export.h>
>  #include <linux/stat.h>
> @@ -711,6 +712,52 @@ const struct sysfs_ops kobj_sysfs_ops =
>  };
>  
>  /**
> + * kobj_completion_init - initialize a kobj_completion object.
> + * @kc: kobj_completion
> + * @ktype: type of kobject to initialize
> + *
> + * kobj_completion structures can be embedded within structures with different
> + * lifetime rules.  During the release of the enclosing object, we can
> + * wait on the release of the kobject so that we don't free it while it's
> + * still busy.
> + */
> +void kobj_completion_init(struct kobj_completion *kc, struct kobj_type *ktype)
> +{
> +	init_completion(&kc->kc_unregister);
> +	kobject_init(&kc->kc_kobj, ktype);
> +}
> +EXPORT_SYMBOL_GPL(kobj_completion_init);
> +
> +/**
> + * kobj_completion_release - release a kobj_completion object
> + * @kobj: kobject embedded in kobj_completion
> + *
> + * Used with kobject_release to notify waiters that the kobject has been
> + * released.
> + */
> +void kobj_completion_release(struct kobject *kobj)
> +{
> +	struct kobj_completion *kc = kobj_to_kobj_completion(kobj);
> +	complete(&kc->kc_unregister);
> +}
> +EXPORT_SYMBOL_GPL(kobj_completion_release);
> +
> +/**
> + * kobj_completion_del_and_wait - release the kobject and wait for it
> + * @kc: kobj_completion object to release
> + *
> + * Delete the kobject from sysfs and  drop the reference count. Then wait
> + * until any outstanding references are also dropped.
> + */
> +void kobj_completion_del_and_wait(struct kobj_completion *kc)
> +{
> +	kobject_del(&kc->kc_kobj);
> +	kobject_put(&kc->kc_kobj);

Why the extra kobject_put() call?  Who added this extra reference to the
object?

thanks,

greg k-h