NFSD: Make nfsd4_remove() wait before returning NFS4ERR_DELAY

nfsd_unlink() can kick off a CB_RECALL (via
vfs_unlink() -> 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_unlink()
again, once.

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

@@ -1788,9 +1788,18 @@ nfsd_unlink(struct svc_rqst *rqstp, struct svc_fh *fhp, int type,
 
 	fh_fill_pre_attrs(fhp);
 	if (type != S_IFDIR) {
+		int retries;
+
 		if (rdentry->d_sb->s_export_op->flags & EXPORT_OP_CLOSE_BEFORE_UNLINK)
 			nfsd_close_cached_files(rdentry);
-		host_err = vfs_unlink(&init_user_ns, dirp, rdentry, NULL);
+
+		for (retries = 1;;) {
+			host_err = vfs_unlink(&init_user_ns, dirp, rdentry, NULL);
+			if (host_err != -EAGAIN || !retries--)
+				break;
+			if (!nfsd_wait_for_delegreturn(rqstp, rinode))
+				break;
+		}
 	} else {
 		host_err = vfs_rmdir(&init_user_ns, dirp, rdentry);
 	}