linux-kernel - Re: [PATCH] kref: minor cleanup

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Date:	Sat, 20 Apr 2013 15:34:13 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Anatol Pomozov <anatol.pomozov@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kref: minor cleanup

On Sat, Apr 20, 2013 at 09:15:10AM -0700, Anatol Pomozov wrote:
> Follow-up for https://lkml.org/lkml/2013/4/12/391

That's not needed in a changelog comment.

> * make warning smp-safe
> * result of atomic _unless_zero functions should be checked by caller
>     to avoid use-after-free error

You also do other things you don't list here.

> 
> Signed-off-by: Anatol Pomozov <anatol.pomozov@...il.com>
> ---
>  include/linux/kobject.h | 1 +
>  include/linux/kref.h    | 9 ++++++---
>  lib/kobject.c           | 2 +-
>  3 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> index 939b112..bfad936 100644
> --- a/include/linux/kobject.h
> +++ b/include/linux/kobject.h
> @@ -101,6 +101,7 @@ extern int __must_check kobject_rename(struct kobject *, const char *new_name);
>  extern int __must_check kobject_move(struct kobject *, struct kobject *);
>  
>  extern struct kobject *kobject_get(struct kobject *kobj);
> +extern struct kobject * __must_check kobject_get_unless_zero(struct kobject *kobj);

Don't add apis that no one uses.

And even if you did, you forgot to export it to modules :(


>  extern void kobject_put(struct kobject *kobj);
>  
>  extern char *kobject_get_path(struct kobject *kobj, gfp_t flag);
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index 4972e6e..817d7f1 100644
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -39,8 +39,11 @@ static inline void kref_init(struct kref *kref)
>   */
>  static inline void kref_get(struct kref *kref)
>  {
> -	WARN_ON(!atomic_read(&kref->refcount));
> -	atomic_inc(&kref->refcount);
> +	/* If refcount was 0 before incrementing then we have a race
> +	 * condition when this kref is freeing by some other thread right now.
> +	 * In this case one should use kref_get_unless_zero()
> +	 */

Did you use scripts/checkpatch.pl?

> +	WARN_ON(atomic_inc_return(&kref->refcount) < 2);

As this is just a warning, I really doubt this type of change is needed,
have you ever hit it?

This is to catch people who forget to initialize a kref before doing the
first kref_get(), it's a help in debugging their code, nothing
"critical".

thanks,

greg k-h
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