Re: [PATCH 3/6] Btrfs: move get root of btrfs_search_slot to a helper

From: Nikolay Borisov <nborisov@suse.com>
To: Liu Bo <bo.liu@linux.alibaba.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 3/6] Btrfs: move get root of btrfs_search_slot to a helper
Date: Wed, 16 May 2018 09:54:58 +0300	[thread overview]
Message-ID: <aef6bb75-af61-3f74-3059-d7900bed1e28@suse.com> (raw)
In-Reply-To: <1526406728-109055-4-git-send-email-bo.liu@linux.alibaba.com>

On 15.05.2018 20:52, Liu Bo wrote:
> It's good to have a helper instead of having all get-root details
> open-coded.
> 
> The new helper locks (if necessary) and sets root node of the path.
> 
> There is no functional change in this commit.
> 
> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>

Codewise it looks ok, you've inverted the conditional so as to give a
more linear flow of the code. I've checked all the various branches and
they seem semantically identical to the open coded version. One minor
nit is that I don't think this is the most suitable name for this
function but I don't really have a better suggestions which would be
more specific.

For the code (the changelog should be more explicit):

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  fs/btrfs/ctree.c | 112 +++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 67 insertions(+), 45 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index a96d308c51b8..399839df7a8f 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -2598,6 +2598,72 @@ int btrfs_find_item(struct btrfs_root *fs_root, struct btrfs_path *path,
>  	return 0;
>  }
>  
> +static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
> +							struct btrfs_path *p,
> +							int write_lock_level)
> +{
> +	struct btrfs_fs_info *fs_info = root->fs_info;
> +	struct extent_buffer *b;
> +	int root_lock;
> +	int level = 0;
> +
> +	/*
> +	 * we try very hard to do read locks on the root
> +	 */
> +	root_lock = BTRFS_READ_LOCK;
> +
> +	if (p->search_commit_root) {
> +		/*
> +		 * the commit roots are read only so we always do read locks
> +		 */
> +		if (p->need_commit_sem)
> +			down_read(&fs_info->commit_root_sem);
> +		b = root->commit_root;
> +		extent_buffer_get(b);
> +		level = btrfs_header_level(b);
> +		if (p->need_commit_sem)
> +			up_read(&fs_info->commit_root_sem);
> +		if (!p->skip_locking)
> +			btrfs_tree_read_lock(b);
> +
> +		goto out;
> +	}
> +
> +	if (p->skip_locking) {
> +		b = btrfs_root_node(root);
> +		level = btrfs_header_level(b);
> +		goto out;
> +	}
> +
> +	/*
> +	 * we don't know the level of the root node until we actually
> +	 * have it read locked
> +	 */
> +	b = btrfs_read_lock_root_node(root);
> +	level = btrfs_header_level(b);
> +	if (level > write_lock_level)
> +		goto out;
> +
> +	/*
> +	 * whoops, must trade for write lock
> +	 */
> +	btrfs_tree_read_unlock(b);
> +	free_extent_buffer(b);
> +	b = btrfs_lock_root_node(root);
> +	root_lock = BTRFS_WRITE_LOCK;
> +	/*
> +	 * the level might have changed, check again
> +	 */
> +	level = btrfs_header_level(b);
> +
> +out:
> +	p->nodes[level] = b;
> +	if (!p->skip_locking)
> +		p->locks[level] = root_lock;
> +	return b;
> +}
> +
> +
>  /*
>   * btrfs_search_slot - look for a key in a tree and perform necessary
>   * modifications to preserve tree invariants.
> @@ -2634,7 +2700,6 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>  	int err;
>  	int level;
>  	int lowest_unlock = 1;
> -	int root_lock;
>  	/* everything at write_lock_level or lower must be write locked */
>  	int write_lock_level = 0;
>  	u8 lowest_level = 0;
> @@ -2672,50 +2737,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
>  
>  again:
>  	prev_cmp = -1;
> -	/*
> -	 * we try very hard to do read locks on the root
> -	 */
> -	root_lock = BTRFS_READ_LOCK;
> -	level = 0;
> -	if (p->search_commit_root) {
> -		/*
> -		 * the commit roots are read only
> -		 * so we always do read locks
> -		 */
> -		if (p->need_commit_sem)
> -			down_read(&fs_info->commit_root_sem);
> -		b = root->commit_root;
> -		extent_buffer_get(b);
> -		level = btrfs_header_level(b);
> -		if (p->need_commit_sem)
> -			up_read(&fs_info->commit_root_sem);
> -		if (!p->skip_locking)
> -			btrfs_tree_read_lock(b);
> -	} else {
> -		if (p->skip_locking) {
> -			b = btrfs_root_node(root);
> -			level = btrfs_header_level(b);
> -		} else {
> -			/* we don't know the level of the root node
> -			 * until we actually have it read locked
> -			 */
> -			b = btrfs_read_lock_root_node(root);
> -			level = btrfs_header_level(b);
> -			if (level <= write_lock_level) {
> -				/* whoops, must trade for write lock */
> -				btrfs_tree_read_unlock(b);
> -				free_extent_buffer(b);
> -				b = btrfs_lock_root_node(root);
> -				root_lock = BTRFS_WRITE_LOCK;
> -
> -				/* the level might have changed, check again */
> -				level = btrfs_header_level(b);
> -			}
> -		}
> -	}
> -	p->nodes[level] = b;
> -	if (!p->skip_locking)
> -		p->locks[level] = root_lock;
> +	b = btrfs_search_slot_get_root(root, p, write_lock_level);
>  
>  	while (b) {
>  		level = btrfs_header_level(b);
> 