Re: [PATCH] vfs: Fix lock inversion in drop_pagecache_sb()

[Fengguang Wu]

On Tue, Mar 25, 2008 at 07:12:27PM +0100, Jan Kara wrote:
> Fix longstanding lock inversion in drop_pagecache_sb by dropping inode_lock
> before calling __invalidate_mapping_pages(). We just have to make sure
> inode won't go away from under us by keeping reference to it and putting
> the reference only after we have safely resumed the scan of the inode
> list. A bit tricky but not too bad...

Reviewed-by: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>

It's a handy trick to iterate through the list_head :-)
I have practiced this in my filecache code, and it works nice.

Fengguang

> Signed-off-by: Jan Kara <jack@xxxxxxx>
> CC: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>
> CC: David Chinner <dgc@xxxxxxx>
> 
> ---
>  fs/drop_caches.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/drop_caches.c b/fs/drop_caches.c
> index 59375ef..f5aae26 100644
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -14,15 +14,21 @@ int sysctl_drop_caches;
>  
>  static void drop_pagecache_sb(struct super_block *sb)
>  {
> -	struct inode *inode;
> +	struct inode *inode, *toput_inode = NULL;
>  
>  	spin_lock(&inode_lock);
>  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
>  		if (inode->i_state & (I_FREEING|I_WILL_FREE))
>  			continue;
> +		__iget(inode);
> +		spin_unlock(&inode_lock);
>  		__invalidate_mapping_pages(inode->i_mapping, 0, -1, true);
> +		iput(toput_inode);
> +		toput_inode = inode;
> +		spin_lock(&inode_lock);
>  	}
>  	spin_unlock(&inode_lock);
> +	iput(toput_inode);
>  }
>  
>  void drop_pagecache(void)
> -- 
> 1.5.2.4
> 

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