From: Al Viro <viro@zeniv.linux.org.uk>
Date: Wed, 10 Oct 2018 20:52:37 +0000 (-0400)
Subject: Document d_splice_alias() calling conventions for ->lookup() users.
X-Git-Tag: v4.20-rc1~121^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1a16dbaf798c5b68ac767444eb045e6f3adaa16d;p=thirdparty%2Fkernel%2Flinux.git

Document d_splice_alias() calling conventions for ->lookup() users.

Short version: it does the right thing when given NULL or ERR_PTR(...)
and its calling conventions are chosen to have minimal PITA when
used in ->lookup() instances.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---

diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 7b7b845c490a4..321d74b739372 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -622,3 +622,14 @@ in your dentry operations instead.
 	alloc_file_clone(file, flags, ops) does not affect any caller's references.
 	On success you get a new struct file sharing the mount/dentry with the
 	original, on failure - ERR_PTR().
+--
+[recommended]
+	->lookup() instances doing an equivalent of
+		if (IS_ERR(inode))
+			return ERR_CAST(inode);
+		return d_splice_alias(inode, dentry);
+	don't need to bother with the check - d_splice_alias() will do the
+	right thing when given ERR_PTR(...) as inode.  Moreover, passing NULL
+	inode to d_splice_alias() will also do the right thing (equivalent of
+	d_add(dentry, NULL); return NULL;), so that kind of special cases
+	also doesn't need a separate treatment.