[PATCH v3 03/19] VFS: allow d_alloc_name() to be used with ->d_hash

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

efivarfs() is similar to other filesystems which use d_alloc_name(), but
it cannot use d_alloc_name() as it has a ->d_hash function.

The only problem with using ->d_hash if available is that it can return
an error, but d_alloc_name() cannot.  If we document that d_alloc_name()
cannot be used when ->d_hash returns an error, then any filesystem which
has a safe ->d_hash can safely use d_alloc_name().

So enhance d_alloc_name() to check for a ->d_hash function
and document that this is not permitted if the ->d_hash function can
fail( which efivarfs_d_hash() cannot).

Also document locking requirements for use.

This is a step towards eventually deprecating d_alloc().

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/dcache.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 789544525c56..d0a504fc62e5 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1945,12 +1945,31 @@ struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
 	return dentry;
 }
 
+/**
+ * d_alloc_name: allocate a dentry for use in a dcache-based filesystem.
+ * @parent: dentry of the parent for the dentry
+ * @name: name of the dentry
+ *
+ * d_alloc_name() allocates a dentry without any protection against
+ * races.  It should only be used in directories that do not support
+ * create/rename/link inode operations and so is particularly suited for
+ * use with simple_dir_inode_operations.  The result is typically passed
+ * to d_make_persistent().
+ *
+ * This must NOT be used by filesystems which provide a d_hash() function
+ * that can return an error.
+ */
 struct dentry *d_alloc_name(struct dentry *parent, const char *name)
 {
 	struct qstr q;
 
 	q.name = name;
 	q.hash_len = hashlen_string(parent, name);
+	if (parent->d_flags & DCACHE_OP_HASH) {
+		int err = parent->d_op->d_hash(parent, &q);
+		if (WARN_ON_ONCE(err))
+			return NULL;
+	}
 	return d_alloc(parent, &q);
 }
 EXPORT_SYMBOL(d_alloc_name);
-- 
2.50.0.107.gf914562f5916.dirty