[PATCH v2 4/9] nfs: use d_splice_alias() in nfs_link()

[NeilBrown <neilb@ownmail.net>]

From: NeilBrown <neil@brown.name>

When filename_linkat() calls filename_create() which ultimately
calls ->lookup, the flags LOOKUP_CREATE|LOOKUP_EXCL are passed.
nfs_lookup() treats this as an exclusive create (which it is) and
skips the ->lookup, leaving the dentry unchanged.

Currently that means nfs_link() can get a hashed dentry (if the name was
already in the cache) or an unhashed dentry (if it wasn't). As none of
d_add(), d_instantiate(), d_splice_alias() could handle both of these,
nfs_link() calls d_drop() and then d_add().

Recent changes to d_splice_alias() mean that it *can* work with either
hashed or unhashed dentries.  Future changes to locking mean that it
will be unsafe to d_drop() a dentry while an operation (in this case
"link()") is still ongoing.

So change to use d_splice_alias(), and not to d_drop() until an error is
detected (as in that case was can't be sure what is actually on the server).

Also update the comment for nfs_is_exclusive_create() to note that
link(), mkdir(), mknod(), symlink() all appear as exclusive creates.
Those other than link() already used d_splice_alias() via
nfs_add_or_obtain().

Possibly nfs_is_exclusive_create() should return true even with NFSv2
when the request isn't an open (not LOOKUP_OPEN).

Signed-off-by: NeilBrown <neil@brown.name>
---
 fs/nfs/dir.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index f219cc1a5fc5..b7cb14310bcf 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1570,6 +1570,9 @@ static int nfs_check_verifier(struct inode *dir, struct dentry *dentry,
 /*
  * Use intent information to check whether or not we're going to do
  * an O_EXCL create using this path component.
+ * Note that link(), mkdir(), mknod(), symlink() all appear as
+ * exclusive creation.  Regular file creation could be distinguished
+ * with LOOKUP_OPEN.
  */
 static int nfs_is_exclusive_create(struct inode *dir, unsigned int flags)
 {
@@ -2699,14 +2702,15 @@ nfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
 		old_dentry, dentry);
 
 	trace_nfs_link_enter(inode, dir, dentry);
-	d_drop(dentry);
 	if (S_ISREG(inode->i_mode))
 		nfs_sync_inode(inode);
 	error = NFS_PROTO(dir)->link(inode, dir, &dentry->d_name);
 	if (error == 0) {
 		nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
 		ihold(inode);
-		d_add(dentry, inode);
+		d_splice_alias(inode, dentry);
+	} else {
+		d_drop(dentry);
 	}
 	trace_nfs_link_exit(inode, dir, dentry, error);
 	return error;
-- 
2.50.0.107.gf914562f5916.dirty