linux-kernel - Re: [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting of stats

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Date:	Mon, 18 Apr 2016 15:39:56 -0400
From:	Waiman Long <waiman.long@....com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	<mingo@...nel.org>, <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH -tip 2/3] locking/pvqspinlock: Avoid double resetting
 of stats

On 04/18/2016 02:31 AM, Davidlohr Bueso wrote:
> ... remove the redundant second iteration, this is most
> likely a copy/past buglet.
>
> Signed-off-by: Davidlohr Bueso<dbueso@...e.de>
> ---
>   kernel/locking/qspinlock_stat.h | 2 --
>   1 file changed, 2 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
> index d734b7502001..72722334237a 100644
> --- a/kernel/locking/qspinlock_stat.h
> +++ b/kernel/locking/qspinlock_stat.h
> @@ -191,8 +191,6 @@ static ssize_t qstat_write(struct file *file, const char __user *user_buf,
>
>   		for (i = 0 ; i<  qstat_num; i++)
>   			WRITE_ONCE(ptr[i], 0);
> -		for (i = 0 ; i<  qstat_num; i++)
> -			WRITE_ONCE(ptr[i], 0);
>   	}
>   	return count;
>   }

The double write is done on purpose. As the statistics count update 
isn't atomic, there is a very small chance (p) that clearing the count 
may happen in the middle of read-modify-write bus transaction. Doing a 
double write will reduce the chance further to p^2. This isn't failsafe, 
but I think is good enough.

However, I don't mind eliminate the double write either as we can always 
view the statistics count after a reset to make sure that they are 
properly cleared.

Cheers,
Longman