linux-kernel - Re: [PATCH] cpuset: mm: Remove memory barrier damage from the page allocator

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Date:	Fri, 2 Mar 2012 10:19:55 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mel Gorman <mgorman@...e.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Miao Xie <miaox@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuset: mm: Remove memory barrier damage from the page
 allocator

On Fri, 2 Mar 2012, Mel Gorman wrote:

> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index e9eaec5..ba6d217 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -92,38 +92,25 @@ extern void cpuset_print_task_mems_allowed(struct task_struct *p);
>   * reading current mems_allowed and mempolicy in the fastpath must protected
>   * by get_mems_allowed()
>   */
> -static inline void get_mems_allowed(void)
> +static inline unsigned long get_mems_allowed(void)
>  {
> -	current->mems_allowed_change_disable++;
> -
> -	/*
> -	 * ensure that reading mems_allowed and mempolicy happens after the
> -	 * update of ->mems_allowed_change_disable.
> -	 *
> -	 * the write-side task finds ->mems_allowed_change_disable is not 0,
> -	 * and knows the read-side task is reading mems_allowed or mempolicy,
> -	 * so it will clear old bits lazily.
> -	 */
> -	smp_mb();
> +	return atomic_read(&current->mems_allowed_seq);
>  }
>
> -static inline void put_mems_allowed(void)
> +/*
> + * If this returns false, the operation that took place after get_mems_allowed
> + * may have failed. It is up to the caller to retry the operation if
> + * appropriate
> + */
> +static inline bool put_mems_allowed(unsigned long seq)
>  {
> -	/*
> -	 * ensure that reading mems_allowed and mempolicy before reducing
> -	 * mems_allowed_change_disable.
> -	 *
> -	 * the write-side task will know that the read-side task is still
> -	 * reading mems_allowed or mempolicy, don't clears old bits in the
> -	 * nodemask.
> -	 */
> -	smp_mb();
> -	--ACCESS_ONCE(current->mems_allowed_change_disable);
> +	return seq == atomic_read(&current->mems_allowed_seq);
>  }

Use seqlock instead of the counter? Seems that you are recoding much of
what a seqlock does. A seqlock also allows you to have a writer that sort
of blocks the reades if necessary.

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