linux-kernel - Re: [PATCH 2/6] percpu-refcount: one bit is enough for REF

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Date:	Wed, 18 Jun 2014 10:37:29 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<cl@...ux-foundation.org>, <kmo@...erainc.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] percpu-refcount: one bit is enough for REF_STATUS

On 06/18/2014 09:08 AM, Tejun Heo wrote:
> percpu-refcount currently reserves two lowest bits of its percpu
> pointer to indicate its state; however, only one bit is used for
> PCPU_REF_DEAD.
> 
> Simplify it by removing PCPU_STATUS_BITS/MASK and testing
> PCPU_REF_DEAD directly.  This also allows the compiler to choose a
> more efficient instruction depending on the architecture.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Kent Overstreet <kmo@...erainc.com>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> ---
>  include/linux/percpu-refcount.h | 4 +---
>  lib/percpu-refcount.c           | 2 +-
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index 5d8920e..bfdeb0d4 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -88,12 +88,10 @@ static inline void percpu_ref_kill(struct percpu_ref *ref)
>  	return percpu_ref_kill_and_confirm(ref, NULL);
>  }
>  
> -#define PCPU_STATUS_BITS	2

Just using "#define PCPU_STATUS_BITS	1" can achieve exactly the same
result as you need. And it is better for future extensions for the status.

Anyway,
Reviewed-by: Lai Jiangshan <laijs@...fujitsu.com>

> -#define PCPU_STATUS_MASK	((1 << PCPU_STATUS_BITS) - 1)
>  #define PCPU_REF_PTR		0
>  #define PCPU_REF_DEAD		1
>  
> -#define REF_STATUS(count)	(((unsigned long) count) & PCPU_STATUS_MASK)
> +#define REF_STATUS(count)	(((unsigned long) count) & PCPU_REF_DEAD)
>  
>  /**
>   * percpu_ref_get - increment a percpu refcount
> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> index 963b703..17bce2b 100644
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -96,7 +96,7 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
>  
>  	/* Mask out PCPU_REF_DEAD */
>  	pcpu_count = (unsigned __percpu *)
> -		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
> +		(((unsigned long) pcpu_count) & ~PCPU_REF_DEAD);
>  
>  	for_each_possible_cpu(cpu)
>  		count += *per_cpu_ptr(pcpu_count, cpu);

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