linux-kernel - Re: [PATCH] mm: remove useless rcu lock-unlock from mapping

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Message-ID: <20110719205541.GQ2357@linux.vnet.ibm.com>
Date:	Tue, 19 Jul 2011 13:55:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove useless rcu lock-unlock from
 mapping_tagged()

On Sun, Jul 17, 2011 at 03:35:23PM +0400, Konstantin Khlebnikov wrote:
> radix_tree_tagged() is lockless, it does not require any protection.

Indeed.  The radix_tree_root structure is a field in the address_space
structure, so no protection is required to get at the field.  If
protection is required on the mapping pointer itself, that would have
to happen in writeback_single_inode() and friends.

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> ---
>  mm/page-writeback.c |    6 +-----
>  1 files changed, 1 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 31f6988..919b45e 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1405,10 +1405,6 @@ EXPORT_SYMBOL(test_set_page_writeback);
>   */
>  int mapping_tagged(struct address_space *mapping, int tag)
>  {
> -	int ret;
> -	rcu_read_lock();
> -	ret = radix_tree_tagged(&mapping->page_tree, tag);
> -	rcu_read_unlock();
> -	return ret;
> +	return radix_tree_tagged(&mapping->page_tree, tag);
>  }
>  EXPORT_SYMBOL(mapping_tagged);
> 
> --
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