Re: [PATCH v5 02/14] VFS: introduce start_dirop() and end_dirop()

On Thu, 2025-11-06 at 11:50 +1100, NeilBrown wrote:
> From: NeilBrown <[email protected]>
> 
> The fact that directory operations (create,remove,rename) are protected
> by a lock on the parent is known widely throughout the kernel.
> In order to change this - to instead lock the target dentry  - it is
> best to centralise this knowledge so it can be changed in one place.
> 
> This patch introduces start_dirop() which is local to VFS code.
> It performs the required locking for create and remove.  Rename
> will be handled separately.
> 
> Various functions with names like start_creating() or start_removing_path(),
> some of which already exist, will export this functionality beyond the VFS.
> 
> end_dirop() is the partner of start_dirop().  It drops the lock and
> releases the reference on the dentry.
> It *is* exported so that various end_creating etc functions can be inline.
> 
> As vfs_mkdir() drops the dentry on error we cannot use end_dirop() as
> that won't unlock when the dentry IS_ERR().  For now we need an explicit
> unlock when dentry IS_ERR().  I hope to change vfs_mkdir() to unlock
> when it drops a dentry so that explicit unlock can go away.
> 
> end_dirop() can always be called on the result of start_dirop(), but not
> after vfs_mkdir().  After a vfs_mkdir() we still may need the explicit
> unlock as seen in end_creating_path().
> 
> As well as adding start_dirop() and end_dirop()
> this patch uses them in:
>  - simple_start_creating (which requires sharing lookup_noperm_common()
>         with libfs.c)
>  - start_removing_path / start_removing_user_path_at
>  - filename_create / end_creating_path()
>  - do_rmdir(), do_unlinkat()
> 
> Reviewed-by: Amir Goldstein <[email protected]>
> Signed-off-by: NeilBrown <[email protected]>
> ---
>  fs/internal.h      |  3 ++
>  fs/libfs.c         | 36 ++++++++---------
>  fs/namei.c         | 98 ++++++++++++++++++++++++++++++++++------------
>  include/linux/fs.h |  2 +
>  4 files changed, 95 insertions(+), 44 deletions(-)
> 
> diff --git a/fs/internal.h b/fs/internal.h
> index 9b2b4d116880..d08d5e2235e9 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -67,6 +67,9 @@ int vfs_tmpfile(struct mnt_idmap *idmap,
>  		const struct path *parentpath,
>  		struct file *file, umode_t mode);
>  struct dentry *d_hash_and_lookup(struct dentry *, struct qstr *);
> +struct dentry *start_dirop(struct dentry *parent, struct qstr *name,
> +			   unsigned int lookup_flags);
> +int lookup_noperm_common(struct qstr *qname, struct dentry *base);
>  
>  /*
>   * namespace.c
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 1661dcb7d983..2d6657947abd 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -2290,27 +2290,25 @@ void stashed_dentry_prune(struct dentry *dentry)
>  	cmpxchg(stashed, dentry, NULL);
>  }
>  
> -/* parent must be held exclusive */
> +/**
> + * simple_start_creating - prepare to create a given name
> + * @parent: directory in which to prepare to create the name
> + * @name:   the name to be created
> + *
> + * Required lock is taken and a lookup in performed prior to creating an
> + * object in a directory.  No permission checking is performed.
> + *
> + * Returns: a negative dentry on which vfs_create() or similar may
> + *  be attempted, or an error.
> + */
>  struct dentry *simple_start_creating(struct dentry *parent, const char *name)
>  {
> -	struct dentry *dentry;
> -	struct inode *dir = d_inode(parent);
> +	struct qstr qname = QSTR(name);
> +	int err;
>  
> -	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;
> +	err = lookup_noperm_common(&qname, parent);
> +	if (err)
> +		return ERR_PTR(err);
> +	return start_dirop(parent, &qname, LOOKUP_CREATE | LOOKUP_EXCL);
>  }
>  EXPORT_SYMBOL(simple_start_creating);
> diff --git a/fs/namei.c b/fs/namei.c
> index 39c4d52f5b54..231e1ffd4b8d 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2765,6 +2765,48 @@ static int filename_parentat(int dfd, struct filename *name,
>  	return __filename_parentat(dfd, name, flags, parent, last, type, NULL);
>  }
>  
> +/**
> + * start_dirop - begin a create or remove dirop, performing locking and lookup
> + * @parent:       the dentry of the parent in which the operation will occur
> + * @name:         a qstr holding the name within that parent
> + * @lookup_flags: intent and other lookup flags.
> + *
> + * The lookup is performed and necessary locks are taken so that, on success,
> + * the returned dentry can be operated on safely.
> + * The qstr must already have the hash value calculated.
> + *
> + * Returns: a locked dentry, or an error.
> + *
> + */
> +struct dentry *start_dirop(struct dentry *parent, struct qstr *name,
> +			   unsigned int lookup_flags)
> +{
> +	struct dentry *dentry;
> +	struct inode *dir = d_inode(parent);
> +
> +	inode_lock_nested(dir, I_MUTEX_PARENT);
> +	dentry = lookup_one_qstr_excl(name, parent, lookup_flags);
> +	if (IS_ERR(dentry))
> +		inode_unlock(dir);
> +	return dentry;
> +}
> +
> +/**
> + * end_dirop - signal completion of a dirop
> + * @de: the dentry which was returned by start_dirop or similar.
> + *
> + * If the de is an error, nothing happens. Otherwise any lock taken to
> + * protect the dentry is dropped and the dentry itself is release (dput()).
> + */
> +void end_dirop(struct dentry *de)
> +{
> +	if (!IS_ERR(de)) {
> +		inode_unlock(de->d_parent->d_inode);
> +		dput(de);
> +	}
> +}
> +EXPORT_SYMBOL(end_dirop);
> +
>  /* does lookup, returns the object with parent locked */
>  static struct dentry *__start_removing_path(int dfd, struct filename *name,
>  					   struct path *path)
> @@ -2781,10 +2823,9 @@ static struct dentry *__start_removing_path(int dfd, struct filename *name,
>  		return ERR_PTR(-EINVAL);
>  	/* don't fail immediately if it's r/o, at least try to report other errors */
>  	error = mnt_want_write(parent_path.mnt);
> -	inode_lock_nested(parent_path.dentry->d_inode, I_MUTEX_PARENT);
> -	d = lookup_one_qstr_excl(&last, parent_path.dentry, 0);
> +	d = start_dirop(parent_path.dentry, &last, 0);
>  	if (IS_ERR(d))
> -		goto unlock;
> +		goto drop;
>  	if (error)
>  		goto fail;
>  	path->dentry = no_free_ptr(parent_path.dentry);
> @@ -2792,10 +2833,9 @@ static struct dentry *__start_removing_path(int dfd, struct filename *name,
>  	return d;
>  
>  fail:
> -	dput(d);
> +	end_dirop(d);
>  	d = ERR_PTR(error);
> -unlock:
> -	inode_unlock(parent_path.dentry->d_inode);
> +drop:
>  	if (!error)
>  		mnt_drop_write(parent_path.mnt);
>  	return d;
> @@ -2910,7 +2950,7 @@ int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
>  }
>  EXPORT_SYMBOL(vfs_path_lookup);
>  
> -static int lookup_noperm_common(struct qstr *qname, struct dentry *base)
> +int lookup_noperm_common(struct qstr *qname, struct dentry *base)
>  {
>  	const char *name = qname->name;
>  	u32 len = qname->len;
> @@ -4223,21 +4263,18 @@ static struct dentry *filename_create(int dfd, struct filename *name,
>  	 */
>  	if (last.name[last.len] && !want_dir)
>  		create_flags &= ~LOOKUP_CREATE;
> -	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
> -	dentry = lookup_one_qstr_excl(&last, path->dentry,
> -				      reval_flag | create_flags);
> +	dentry = start_dirop(path->dentry, &last, reval_flag | create_flags);
>  	if (IS_ERR(dentry))
> -		goto unlock;
> +		goto out_drop_write;
>  
>  	if (unlikely(error))
>  		goto fail;
>  
>  	return dentry;
>  fail:
> -	dput(dentry);
> +	end_dirop(dentry);
>  	dentry = ERR_PTR(error);
> -unlock:
> -	inode_unlock(path->dentry->d_inode);
> +out_drop_write:
>  	if (!error)
>  		mnt_drop_write(path->mnt);
>  out:
> @@ -4256,11 +4293,26 @@ struct dentry *start_creating_path(int dfd, const char *pathname,
>  }
>  EXPORT_SYMBOL(start_creating_path);
>  
> +/**
> + * end_creating_path - finish a code section started by start_creating_path()
> + * @path: the path instantiated by start_creating_path()
> + * @dentry: the dentry returned by start_creating_path()
> + *
> + * end_creating_path() will unlock and locks taken by start_creating_path()
> + * and drop an references that were taken.  It should only be called
> + * if start_creating_path() returned a non-error.
> + * If vfs_mkdir() was called and it returned an error, that error *should*
> + * be passed to end_creating_path() together with the path.
> + */
>  void end_creating_path(const struct path *path, struct dentry *dentry)
>  {
> -	if (!IS_ERR(dentry))
> -		dput(dentry);
> -	inode_unlock(path->dentry->d_inode);
> +	if (IS_ERR(dentry))
> +		/* The parent is still locked despite the error from
> +		 * vfs_mkdir() - must unlock it.
> +		 */
> +		inode_unlock(path->dentry->d_inode);
> +	else
> +		end_dirop(dentry);
>  	mnt_drop_write(path->mnt);
>  	path_put(path);
>  }
> @@ -4592,8 +4644,7 @@ int do_rmdir(int dfd, struct filename *name)
>  	if (error)
>  		goto exit2;
>  
> -	inode_lock_nested(path.dentry->d_inode, I_MUTEX_PARENT);
> -	dentry = lookup_one_qstr_excl(&last, path.dentry, lookup_flags);
> +	dentry = start_dirop(path.dentry, &last, lookup_flags);
>  	error = PTR_ERR(dentry);
>  	if (IS_ERR(dentry))
>  		goto exit3;
> @@ -4602,9 +4653,8 @@ int do_rmdir(int dfd, struct filename *name)
>  		goto exit4;
>  	error = vfs_rmdir(mnt_idmap(path.mnt), path.dentry->d_inode, dentry);
>  exit4:
> -	dput(dentry);
> +	end_dirop(dentry);
>  exit3:
> -	inode_unlock(path.dentry->d_inode);
>  	mnt_drop_write(path.mnt);
>  exit2:
>  	path_put(&path);
> @@ -4721,8 +4771,7 @@ int do_unlinkat(int dfd, struct filename *name)
>  	if (error)
>  		goto exit2;
>  retry_deleg:
> -	inode_lock_nested(path.dentry->d_inode, I_MUTEX_PARENT);
> -	dentry = lookup_one_qstr_excl(&last, path.dentry, lookup_flags);
> +	dentry = start_dirop(path.dentry, &last, lookup_flags);
>  	error = PTR_ERR(dentry);
>  	if (!IS_ERR(dentry)) {
>  
> @@ -4737,9 +4786,8 @@ int do_unlinkat(int dfd, struct filename *name)
>  		error = vfs_unlink(mnt_idmap(path.mnt), path.dentry->d_inode,
>  				   dentry, &delegated_inode);
>  exit3:
> -		dput(dentry);
> +		end_dirop(dentry);
>  	}
> -	inode_unlock(path.dentry->d_inode);
>  	if (inode)
>  		iput(inode);	/* truncate the inode here */
>  	inode = NULL;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 03e450dd5211..9e7556e79d19 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3196,6 +3196,8 @@ extern void iterate_supers_type(struct file_system_type *,
>  void filesystems_freeze(void);
>  void filesystems_thaw(void);
>  
> +void end_dirop(struct dentry *de);
> +
>  extern int dcache_dir_open(struct inode *, struct file *);
>  extern int dcache_dir_close(struct inode *, struct file *);
>  extern loff_t dcache_dir_lseek(struct file *, loff_t, int);

Reviewed-by: Jeff Layton <[email protected]>