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

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Date:	Fri, 19 Apr 2013 19:24:15 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Anatol Pomozov <anatol.pomozov@...il.com>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH] kref: minor cleanup

On Fri, Apr 19, 2013 at 06:33:54PM -0700, Anatol Pomozov wrote:
> Follow-up for https://lkml.org/lkml/2013/4/12/391
> 
> * make warning smp-safe
> * result of atomic _unless_zero functions should be checked by caller
>     to avoid use-after-free error
> 
> Signed-off-by: Anatol Pomozov <anatol.pomozov@...il.com>
> ---
>  include/linux/kref.h | 9 ++++++---
>  lib/kobject.c        | 3 ++-
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index 4972e6e..092529a 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 freing by some other thread right now.
> +	 * In this case one should use kref_get_unless_zero()
> +	 */
> +	WARN_ON(atomic_inc_return(&kref->refcount) < 2);

What happens if you disable WARN_ON(), does the atomic_inc_return() go
away as well?  Or did we fix that?

>  }
>  
>  /**
> @@ -100,7 +103,7 @@ static inline int kref_put_mutex(struct kref *kref,
>  				 struct mutex *lock)
>  {
>  	WARN_ON(release == NULL);
> -        if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
> +	if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
>  		mutex_lock(lock);
>  		if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
>  			mutex_unlock(lock);
> diff --git a/lib/kobject.c b/lib/kobject.c
> index a654866..bbd7362 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -529,7 +529,8 @@ struct kobject *kobject_get(struct kobject *kobj)
>  	return kobj;
>  }
>  
> -static struct kobject *kobject_get_unless_zero(struct kobject *kobj)
> +static struct kobject *__must_check kobject_get_unless_zero(
> +		struct kobject *kobj)

__must_check needs to be in the .h file, not the .c file.

thanks,

greg k-h
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