Re: [PATCH 02/17] new helper: simple_start_creating()

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2025-06-13 at 08:34 +0100, Al Viro wrote:
> Set the things up for kernel-initiated creation of object in
> a tree-in-dcache filesystem.  With respect to locking it's
> an equivalent of filename_create() - we either get a negative
> dentry with locked parent, or ERR_PTR() and no locks taken.
> 
> tracefs and debugfs had that open-coded as part of their
> object creation machinery; switched to calling new helper.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/debugfs/inode.c | 21 ++-------------------
>  fs/libfs.c         | 25 +++++++++++++++++++++++++
>  fs/tracefs/inode.c | 15 ++-------------
>  include/linux/fs.h |  1 +
>  4 files changed, 30 insertions(+), 32 deletions(-)
> 
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index 30c4944e1862..08638e39bd12 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -384,26 +384,9 @@ static struct dentry *start_creating(const char *name, struct dentry *parent)
>  	if (!parent)
>  		parent = debugfs_mount->mnt_root;
>  
> -	inode_lock(d_inode(parent));
> -	if (unlikely(IS_DEADDIR(d_inode(parent))))
> -		dentry = ERR_PTR(-ENOENT);
> -	else
> -		dentry = lookup_noperm(&QSTR(name), parent);
> -	if (!IS_ERR(dentry) && d_really_is_positive(dentry)) {
> -		if (d_is_dir(dentry))
> -			pr_err("Directory '%s' with parent '%s' already present!\n",
> -			       name, parent->d_name.name);
> -		else
> -			pr_err("File '%s' in directory '%s' already present!\n",
> -			       name, parent->d_name.name);

Any chance we could keep a pr_err() for this case? I was doing some
debugfs work recently, and found it helpful.

> -		dput(dentry);
> -		dentry = ERR_PTR(-EEXIST);
> -	}
> -
> -	if (IS_ERR(dentry)) {
> -		inode_unlock(d_inode(parent));
> +	dentry = simple_start_creating(parent, name);
> +	if (IS_ERR(dentry))
>  		simple_release_fs(&debugfs_mount, &debugfs_mount_count);
> -	}
>  
>  	return dentry;
>  }
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 42e226af6095..833ad5ed10f5 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -2260,3 +2260,28 @@ void stashed_dentry_prune(struct dentry *dentry)
>  	 */
>  	cmpxchg(stashed, dentry, NULL);
>  }
> +
> +/* parent must be held exclusive */
> +struct dentry *simple_start_creating(struct dentry *parent, const char *name)
> +{
> +	struct dentry *dentry;
> +	struct inode *dir = d_inode(parent);
> +
> +	inode_lock(dir);
> +	if (unlikely(IS_DEADDIR(dir))) {
> +		inode_unlock(dir);
> +		return ERR_PTR(-ENOENT);
> +	}
> +	dentry = lookup_noperm(&QSTR(name), parent);
> +	if (IS_ERR(dentry)) {
> +		inode_unlock(dir);
> +		return dentry;
> +	}
> +	if (dentry->d_inode) {
> +		dput(dentry);
> +		inode_unlock(dir);
> +		return ERR_PTR(-EEXIST);
> +	}
> +	return dentry;
> +}
> +EXPORT_SYMBOL(simple_start_creating);
> diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
> index a3fd3cc591bd..4e5d091e9263 100644
> --- a/fs/tracefs/inode.c
> +++ b/fs/tracefs/inode.c
> @@ -551,20 +551,9 @@ struct dentry *tracefs_start_creating(const char *name, struct dentry *parent)
>  	if (!parent)
>  		parent = tracefs_mount->mnt_root;
>  
> -	inode_lock(d_inode(parent));
> -	if (unlikely(IS_DEADDIR(d_inode(parent))))
> -		dentry = ERR_PTR(-ENOENT);
> -	else
> -		dentry = lookup_noperm(&QSTR(name), parent);
> -	if (!IS_ERR(dentry) && d_inode(dentry)) {
> -		dput(dentry);
> -		dentry = ERR_PTR(-EEXIST);
> -	}
> -
> -	if (IS_ERR(dentry)) {
> -		inode_unlock(d_inode(parent));
> +	dentry = simple_start_creating(parent, name);
> +	if (IS_ERR(dentry))
>  		simple_release_fs(&tracefs_mount, &tracefs_mount_count);
> -	}
>  
>  	return dentry;
>  }
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 96c7925a6551..9f75f8836bbd 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3619,6 +3619,7 @@ extern int simple_fill_super(struct super_block *, unsigned long,
>  			     const struct tree_descr *);
>  extern int simple_pin_fs(struct file_system_type *, struct vfsmount **mount, int *count);
>  extern void simple_release_fs(struct vfsmount **mount, int *count);
> +struct dentry *simple_start_creating(struct dentry *, const char *);
>  
>  extern ssize_t simple_read_from_buffer(void __user *to, size_t count,
>  			loff_t *ppos, const void *from, size_t available);

-- 
Jeff Layton <jlayton@kernel.org>