[v4,3/9] net: xfrm: use __this_cpu_read per-cpu helper

From: Shan Wei <davidshan@tencent.com>


Signed-off-by: Shan Wei <davidshan@tencent.com>
---
v4:
  derefrence pointer before reading to avoid compile warning.
---
 net/xfrm/xfrm_ipcomp.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

On Tue, Nov 13, 2012 at 09:52:09AM +0800, Shan Wei wrote:
> From: Shan Wei <davidshan@tencent.com>
> 

Please add a proper commit message, explaining why you do this change.

> 
> Signed-off-by: Shan Wei <davidshan@tencent.com>
> ---
> v4:
>   derefrence pointer before reading to avoid compile warning.
> ---
>  net/xfrm/xfrm_ipcomp.c |    8 +++-----
>  1 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
> index e5246fb..2906d52 100644
> --- a/net/xfrm/xfrm_ipcomp.c
> +++ b/net/xfrm/xfrm_ipcomp.c
> @@ -276,18 +276,16 @@ static struct crypto_comp * __percpu *ipcomp_alloc_tfms(const char *alg_name)
>  	struct crypto_comp * __percpu *tfms;
>  	int cpu;
>  
> -	/* This can be any valid CPU ID so we don't need locking. */
> -	cpu = raw_smp_processor_id();
>  
>  	list_for_each_entry(pos, &ipcomp_tfms_list, list) {
>  		struct crypto_comp *tfm;
>  
> -		tfms = pos->tfms;
> -		tfm = *per_cpu_ptr(tfms, cpu);
> +		/* This can be any valid CPU ID so we don't need locking. */
> +		tfm = __this_cpu_read(*pos->tfms);

This should just fetch the tfm pointer, so why exactly __this_cpu_read
is better than __this_cpu_ptr? Please keep in mind that performance is
not the most important thing here. It's much more important that it
works in any case.
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Steffen Klassert said, at 2012/11/13 15:21:
> 
> This should just fetch the tfm pointer, so why exactly __this_cpu_read
> is better than __this_cpu_ptr? Please keep in mind that performance is
> not the most important thing here. It's much more important that it
> works in any case.
 
[0/9] describes why we should submit this series patches.
you are not included in original mail.

this_cpu_ptr relocates and address. this_cpu_read() relocates the address
and performs the fetch. If you want to operate on rda(defined as per_cpu) 
then you can only use this_cpu_ptr. this_cpu_read() saves you more instructions
since it can do the relocation and the fetch in one instruction.

More info, please refer to http://www.spinics.net/lists/kernel/msg1435153.html

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On Tue, Nov 13, 2012 at 05:33:19PM +0800, Shan Wei wrote:
> Steffen Klassert said, at 2012/11/13 15:21:
> > 
> > This should just fetch the tfm pointer, so why exactly __this_cpu_read
> > is better than __this_cpu_ptr? Please keep in mind that performance is
> > not the most important thing here. It's much more important that it
> > works in any case.
>  
> [0/9] describes why we should submit this series patches.
> you are not included in original mail.
> 
> this_cpu_ptr relocates and address. this_cpu_read() relocates the address
> and performs the fetch. If you want to operate on rda(defined as per_cpu) 
> then you can only use this_cpu_ptr. this_cpu_read() saves you more instructions
> since it can do the relocation and the fetch in one instruction.
> 

Ok, so please add a commit message to describe your changes.

Thanks.
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