[PATCH v4 3/7] VFS: enhance d_splice_alias() to handle in-lookup dentries

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

We currently have three interfaces for attaching existing inodes to
normal filesystems(*).
- d_add() requires an unhashed or in-lookup dentry and doesn't handle
  splicing in case a directory already has dentry
- d_instantiate() requires a hashed dentry, and also doesn't handle
  splicing.
- d_splice_alias() requires unhashed or in-lookup and does handle
  splicing, and can return an alternate dentry.

So there is no interface that supports both hashed and in-lookup, which
is what ->atomic_open needs to deal with.

Some filesystems check for in-lookup in their atomic_open and if found,
perform a ->lookup and can subsequently use d_instantiate() if the
dentry is still negative.  Others d_drop() the dentry so they can use
d_splice_alias().

This last will cause a problem for proposed changes to locking which
require the dentry to remain hashed while and operation proceeds on it.

There is also no interface which splices a directory (which might
already have a dentry) to a hashed dentry.  Filesystems which need to do
this d_drop() first.

Some filesystemss (NFS) skip ->lookup processing for
  LOOKUP_CREATE|LOOKUP_EXCL
which includes mknod, link, symlink etc.  So these inode operations
might get an unhashed or a hashed-negative dentry.  There is no
interface for instantiating these so again they need to unhash
first (nfs_link)

So with this patch d_splice_alias() can handle hashed, unhashed, or
in-lookup dentries.  This makes it suitable for ->lookup, ->atomic_open,
and ->mkdir and others.

As a side effect d_add() will also now handle hashed dentries, but
I have plans to remove d_add() as there is no benefit having it as
well as the others.

__d_add() currently contains code that is identical to
__d_instantiate(), so the former is changed to call the later, and both
d_add() and d_instantiate() call __d_add().

* There is also d_make_persistent() for filesystems which are
  dcache-based and don't support mkdir, create etc, and
  d_instantiate_new() for newly created inodes that are still locked.

Signed-off-by: NeilBrown <neil@brown.name>
---
 Documentation/filesystems/vfs.rst |  4 ++--
 fs/dcache.c                       | 31 ++++++++++++-------------------
 2 files changed, 14 insertions(+), 21 deletions(-)

diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 7c753148af88..d8df0a84cdba 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -507,8 +507,8 @@ otherwise noted.
 	dentry before the first mkdir returns.
 
 	If there is any chance this could happen, then the new inode
-	should be d_drop()ed and attached with d_splice_alias().  The
-	returned dentry (if any) should be returned by ->mkdir().
+	should be attached with d_splice_alias().  The returned
+	dentry (if any) should be returned by ->mkdir().
 
 ``rmdir``
 	called by the rmdir(2) system call.  Only required if you want
diff --git a/fs/dcache.c b/fs/dcache.c
index 1a065df22ecd..a4ee380f3dfc 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2068,7 +2068,6 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
  * (or otherwise set) by the caller to indicate that it is now
  * in use by the dcache.
  */
- 
 void d_instantiate(struct dentry *entry, struct inode * inode)
 {
 	BUG_ON(d_really_is_positive(entry));
@@ -2829,18 +2828,14 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode,
 		dir = dentry->d_parent->d_inode;
 		n = start_dir_add(dir);
 		__d_lookup_unhash(dentry);
+		__d_rehash(dentry);
+	} else if (d_unhashed(dentry)) {
+		__d_rehash(dentry);
 	}
 	if (unlikely(ops))
 		d_set_d_op(dentry, ops);
-	if (inode) {
-		unsigned add_flags = d_flags_for_inode(inode);
-		hlist_add_head(&dentry->d_alias, &inode->i_dentry);
-		raw_write_seqcount_begin(&dentry->d_seq);
-		__d_set_inode_and_type(dentry, inode, add_flags);
-		raw_write_seqcount_end(&dentry->d_seq);
-		fsnotify_update_flags(dentry);
-	}
-	__d_rehash(dentry);
+	if (inode)
+		__d_instantiate(dentry, inode);
 	if (dir) {
 		end_dir_add(dir, n);
 		__d_wake_in_lookup_waiters(dentry);
@@ -3142,8 +3137,6 @@ struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
 	if (IS_ERR(inode))
 		return ERR_CAST(inode);
 
-	BUG_ON(!d_unhashed(dentry));
-
 	if (!inode)
 		goto out;
 
@@ -3192,6 +3185,8 @@ struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
  * @inode:  the inode which may have a disconnected dentry
  * @dentry: a negative dentry which we want to point to the inode.
  *
+ * @dentry must be negative and may be in-lookup or unhashed or hashed.
+ *
  * If inode is a directory and has an IS_ROOT alias, then d_move that in
  * place of the given dentry and return it, else simply d_add the inode
  * to the dentry and return NULL.
@@ -3199,16 +3194,14 @@ struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
  * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
  * we should error out: directories can't have multiple aliases.
  *
- * This is needed in the lookup routine of any filesystem that is exportable
- * (via knfsd) so that we can build dcache paths to directories effectively.
+ * This should be used to return the result of ->lookup() and to
+ * instantiate the result of ->mkdir(), is often useful for
+ * ->atomic_open, and may be used to instantiate other objects.
  *
  * If a dentry was found and moved, then it is returned.  Otherwise NULL
- * is returned.  This matches the expected return value of ->lookup.
+ * is returned.  This matches the expected return value of ->lookup and
+ * ->mkdir.
  *
- * Cluster filesystems may call this function with a negative, hashed dentry.
- * In that case, we know that the inode will be a regular file, and also this
- * will only occur during atomic_open. So we need to check for the dentry
- * being already hashed only in the final case.
  */
 struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 {
-- 
2.50.0.107.gf914562f5916.dirty