linux-kernel - Re: [PATCH 1/2] radix-tree: clear all tags in radix_tree_node_rcu

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Message-ID: <20100820080039.GB3777@amd>
Date:	Fri, 20 Aug 2010 18:00:39 +1000
From:	Nick Piggin <npiggin@...nel.dk>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	jack@...e.cz, npiggin@...nel.dk
Subject: Re: [PATCH 1/2] radix-tree: clear all tags in
 radix_tree_node_rcu_free

On Fri, Aug 20, 2010 at 03:22:06PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@...hat.com>
> 
> Commit f446daaea9d4a420d16c606f755f3689dcb2d0ce ("mm: implement
> writeback livelock avoidance using page tagging") introduced a new
> radix tree tag, increasing the number of tags in each node from 2 to
> 3. It did not, however, fix up the code in
> radix_tree_node_rcu_free() that cleans up after radix_tree_shrink()
> and hence could leave stray tags set in the new tag array.
> 
> The result is that the livelock avoidance code added in the the
> above commit would hit stale tags when doing tag based lookups,
> resulting in livelocks when trying to traverse the tree.
> 
> Fix this problem in radix_tree_node_rcu_free() so it doesn't happen
> again in the future by using a loop to walk all the tags up to
> RADIX_TREE_MAX_TAGS to clear the stray tags radix_tree_shrink()
> leaves behind.
> 
> Signed-off-by: Dave Chinner <dchinner@...hat.com>

Acked-by: Nick Piggin <npiggin@...nel.dk>

> ---
>  lib/radix-tree.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index e907858..1014171 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -174,14 +174,16 @@ static void radix_tree_node_rcu_free(struct rcu_head *head)
>  {
>  	struct radix_tree_node *node =
>  			container_of(head, struct radix_tree_node, rcu_head);
> +	int i;
>  
>  	/*
>  	 * must only free zeroed nodes into the slab. radix_tree_shrink
>  	 * can leave us with a non-NULL entry in the first slot, so clear
>  	 * that here to make sure.
>  	 */
> -	tag_clear(node, 0, 0);
> -	tag_clear(node, 1, 0);
> +	for (i = 0; i < RADIX_TREE_MAX_TAGS; i++)
> +		tag_clear(node, i, 0);
> +
>  	node->slots[0] = NULL;
>  	node->count = 0;
>  
> -- 
> 1.7.1
--
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