NFSD: Make nfsd4_rename() wait before returning NFS4ERR_DELAY

nfsd_rename() can kick off a CB_RECALL (via
vfs_rename() -> leases_conflict()) if a delegation is present.
Before returning NFS4ERR_DELAY, give the client holding that
delegation a chance to return it and then retry the nfsd_rename()
again, once.

This version of the patch handles renaming an existing file,
but does not deal with renaming onto an existing file. That
case will still always trigger an NFS4ERR_DELAY.

Link: https://bugzilla.linux-nfs.org/show_bug.cgi?id=354
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/nfsd/vfs.c

@@ -1696,7 +1696,15 @@ nfsd_rename(struct svc_rqst *rqstp, struct svc_fh *ffhp, char *fname, int flen,
 			.new_dir	= tdir,
 			.new_dentry	= ndentry,
 		};
-		host_err = vfs_rename(&rd);
+		int retries;
+
+		for (retries = 1;;) {
+			host_err = vfs_rename(&rd);
+			if (host_err != -EAGAIN || !retries--)
+				break;
+			if (!nfsd_wait_for_delegreturn(rqstp, d_inode(odentry)))
+				break;
+		}
 		if (!host_err) {
 			host_err = commit_metadata(tfhp);
 			if (!host_err)