linux-kernel - Re: [PATCH 2/3] tracing/stat: simplify rbtree freeing code

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Date:	Mon, 25 May 2009 17:59:40 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] tracing/stat: simplify rbtree freeing code

On Mon, May 25, 2009 at 04:46:29PM +0800, Li Zefan wrote:
> When closing a trace_stat file, we destroy the rbtree constructed during
> file open, but there is memory leak that the root node is not freed:
> 
> static struct rb_node *release_next(struct rb_node *node)
> {
> 	...
> 	else {
> 		if (!parent)		<-- here we leak @node
> 			return NULL;
> 	...
> }
> 
> This patch keeps removing root node until the tree is empty. We regress
> from O(n) to O(nlogn), but since both open() and read() are O(nlogn) and
> it's a slow path, this change won't affect scalibility.
> 
> [ Impact: fix memory leak when closing a trace_stat file ]
> 
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  kernel/trace/trace_stat.c |   39 +++++----------------------------------
>  1 files changed, 5 insertions(+), 34 deletions(-)
> 
> diff --git a/kernel/trace/trace_stat.c b/kernel/trace/trace_stat.c
> index 6efbcb4..ed18701 100644
> --- a/kernel/trace/trace_stat.c
> +++ b/kernel/trace/trace_stat.c
> @@ -42,47 +42,18 @@ static DEFINE_MUTEX(all_stat_sessions_mutex);
>  /* The root directory for all stat files */
>  static struct dentry		*stat_dir;
>  
> -/*
> - * Iterate through the rbtree using a post order traversal path
> - * to release the next node.
> - * It won't necessary release one at each iteration
> - * but it will at least advance closer to the next one
> - * to be released.
> - */
> -static struct rb_node *release_next(struct rb_node *node)
> +static void reset_stat_session(struct stat_session *session)
>  {
>  	struct stat_node *snode;
> -	struct rb_node *parent = rb_parent(node);
> -
> -	if (node->rb_left)
> -		return node->rb_left;
> -	else if (node->rb_right)
> -		return node->rb_right;
> -	else {
> -		if (!parent)
> -			return NULL;
> -		if (parent->rb_left == node)
> -			parent->rb_left = NULL;
> -		else
> -			parent->rb_right = NULL;
> +	struct rb_root *sroot = &session->stat_root;
>  
> -		snode = container_of(node, struct stat_node, node);
> +	while (!RB_EMPTY_ROOT(sroot)) {
> +		snode = rb_entry(sroot->rb_node, struct stat_node, node);
> +		rb_erase(&snode->node, sroot);



Why not just keep the previous version but sligthly
modified:


	while (node)
		node = release_next(node);

	if (!RB_EMPTY_ROOT(root)) {
		node = rb_entry(...)
		kfree(....)
		root = RB_ROOT
	}

Because the problem with rb_erase() is the wasteful color based rebalancing
performed whereas here we just need to walk into the tree and free
the nodes.

Hm?

Frederic.


>  		kfree(snode);
> -
> -		return parent;
>  	}
>  }
>  
> -static void reset_stat_session(struct stat_session *session)
> -{
> -	struct rb_node *node = session->stat_root.rb_node;
> -
> -	while (node)
> -		node = release_next(node);
> -
> -	session->stat_root = RB_ROOT;
> -}
> -
>  static void destroy_session(struct stat_session *session)
>  {
>  	debugfs_remove(session->file);
> -- 
> 1.5.4.rc3
> 

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