linux-kernel - Re: [PATCH 09/18] fs: rework icount to be a locked variable

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Date:	Fri, 8 Oct 2010 03:27:49 -0400
From:	Christoph Hellwig <hch@....de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	chris.mason@...cle.com, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 09/18] fs: rework icount to be a locked variable

> index 2953e9f..9f04478 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -1964,8 +1964,14 @@ void btrfs_add_delayed_iput(struct inode *inode)
>  	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
>  	struct delayed_iput *delayed;
>  
> -	if (atomic_add_unless(&inode->i_count, -1, 1))
> +	/* XXX: filesystems should not play refcount games like this */
> +	spin_lock(&inode->i_lock);
> +	if (inode->i_ref > 1) {
> +		inode->i_ref--;
> +		spin_unlock(&inode->i_lock);
>  		return;
> +	}
> +	spin_unlock(&inode->i_lock);

Yeah, all that i_count/i_ref mess in btrfs needs some serious work.
Chris?

> +
> +/*
> + * inode_lock must be held
> + */
> +void iref_locked(struct inode *inode)
> +{
> +	inode->i_ref++;
> +}
>  EXPORT_SYMBOL_GPL(iref_locked);

I'm a big fan of _GPL exports, but adding this for a trivial counter
increment seems a bit weird. 

>  int iref_read(struct inode *inode)
>  {
> -	return atomic_read(&inode->i_count);
> +	int ref;
> +
> +	spin_lock(&inode->i_lock);
> +	ref = inode->i_ref;
> +	spin_unlock(&inode->i_lock);
> +	return ref;
>  }

There's no need to lock a normal 32-bit variable for readers.

> +		inode->i_ref--;
> +		if (inode->i_ref == 0) {

		if (--inode->i_ref == 0) {

might be a bit more idiomatic.

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