linux-kernel - Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of this_cpu

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Message-ID: <5751F48A.6090508@hpe.com>
Date:	Fri, 3 Jun 2016 17:20:10 -0400
From:	Waiman Long <waiman.long@....com>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
	<peterz@...radead.org>
Subject: Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of this_cpu_dec

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@...ux.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