new helper: simple_start_creating()

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.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/debugfs/inode.c

@@ -384,27 +384,12 @@ 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);
-		dput(dentry);
-		dentry = ERR_PTR(-EEXIST);
-	}
-
+	dentry = simple_start_creating(parent, name);
 	if (IS_ERR(dentry)) {
-		inode_unlock(d_inode(parent));
+		if (dentry == ERR_PTR(-EEXIST))
+			pr_err("'%s' already exists in '%pd'\n", name, parent);
 		simple_release_fs(&debugfs_mount, &debugfs_mount_count);
 	}
-
 	return dentry;
 }
 

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);

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;
 }

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);