linux-kernel - Re: [PATCH] sysfs: use atomic_inc_unless_negative in sysfs_get

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Date:	Mon, 18 Mar 2013 17:14:18 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Maarten Lankhorst <m.b.lankhorst@...il.com>, tj@...nel.org
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: use atomic_inc_unless_negative in sysfs_get_active

On Fri, Mar 08, 2013 at 04:07:27PM +0100, Maarten Lankhorst wrote:
> It seems that sysfs has an interesting way of doing the same thing.
> This removes the cpu_relax unfortunately, but if it's really needed,
> it would be better to add this to include/linux/atomic.h to benefit
> all atomic ops users.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@...onical.com>
>  ---

Tejun wrote this code originally, so I'll defer to him as to if the
patch below is a valid cleanup or not.

Tejun?


> 
> diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
> index 2fbdff6..7f968ed 100644
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -165,21 +165,8 @@ struct sysfs_dirent *sysfs_get_active(struct sysfs_dirent *sd)
>  	if (unlikely(!sd))
>  		return NULL;
>  
> -	while (1) {
> -		int v, t;
> -
> -		v = atomic_read(&sd->s_active);
> -		if (unlikely(v < 0))
> -			return NULL;
> -
> -		t = atomic_cmpxchg(&sd->s_active, v, v + 1);
> -		if (likely(t == v))
> -			break;
> -		if (t < 0)
> -			return NULL;
> -
> -		cpu_relax();
> -	}
> +	if (!atomic_inc_unless_negative(&sd->s_active))
> +		return NULL;
>  
>  	if (likely(!ignore_lockdep(sd)))
>  		rwsem_acquire_read(&sd->dep_map, 0, 1, _RET_IP_);
--
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