Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of this_cpu_dec

[Waiman Long <waiman.long@hpe.com>]

On 06/03/2016 05:48 AM, Pan Xinhui wrote:
> queued_spin_lock_slowpath should not worry about interrupt change
> node->count by accident because ->count is inc and dec when we
> enter/leave queued_spin_lock_slowpath.
>
> So this_cpu_dec() does some no point things here, lets use this_cpu_ptr
> for a small optimization.
>
> Signed-off-by: Pan Xinhui<xinhui.pan@linux.vnet.ibm.com>
> ---
>   kernel/locking/qspinlock.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 99f31e4..2b4daac 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -492,7 +492,7 @@ release:
>   	/*
>   	 * release the node
>   	 */
> -	this_cpu_dec(mcs_nodes[0].count);
> +	this_cpu_ptr(&mcs_nodes[0])->count--;
>   }
>   EXPORT_SYMBOL(queued_spin_lock_slowpath);
>

Is this going to generate better code for PPC? For x86, I think it will 
cause more instruction to be issued.

Cheers,
Longman