NFSv4: Return NFS4ERR_DELAY when a delegation recall fails due to igrab()

If the attempt to recall the delegation fails because the inode is
in the process of being evicted from cache, then use NFS4ERR_DELAY
to ask the server to retry later.
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/callback_proc.c

@@ -40,7 +40,9 @@ __be32 nfs4_callback_getattr(void *argp, void *resp,
 		rpc_peeraddr2str(cps->clp->cl_rpcclient, RPC_DISPLAY_ADDR));
 
 	inode = nfs_delegation_find_inode(cps->clp, &args->fh);
-	if (inode == NULL) {
+	if (IS_ERR(inode)) {
+		if (inode == ERR_PTR(-EAGAIN))
+			res->status = htonl(NFS4ERR_DELAY);
 		trace_nfs4_cb_getattr(cps->clp, &args->fh, NULL,
 				-ntohl(res->status));
 		goto out;
@@ -86,7 +88,9 @@ __be32 nfs4_callback_recall(void *argp, void *resp,
 
 	res = htonl(NFS4ERR_BADHANDLE);
 	inode = nfs_delegation_find_inode(cps->clp, &args->fh);
-	if (inode == NULL) {
+	if (IS_ERR(inode)) {
+		if (inode == ERR_PTR(-EAGAIN))
+			res = htonl(NFS4ERR_DELAY);
 		trace_nfs4_cb_recall(cps->clp, &args->fh, NULL,
 				&args->stateid, -ntohl(res));
 		goto out;

fs/nfs/delegation.c

@@ -856,12 +856,14 @@ nfs_delegation_find_inode_server(struct nfs_server *server,
 		if (delegation->inode != NULL &&
 		    nfs_compare_fh(fhandle, &NFS_I(delegation->inode)->fh) == 0) {
 			res = igrab(delegation->inode);
+			spin_unlock(&delegation->lock);
+			if (res != NULL)
+				return res;
+			return ERR_PTR(-EAGAIN);
 		}
 		spin_unlock(&delegation->lock);
-		if (res != NULL)
-			break;
 	}
-	return res;
+	return ERR_PTR(-ENOENT);
 }
 
 /**
@@ -876,16 +878,16 @@ struct inode *nfs_delegation_find_inode(struct nfs_client *clp,
 					const struct nfs_fh *fhandle)
 {
 	struct nfs_server *server;
-	struct inode *res = NULL;
+	struct inode *res;
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(server, &clp->cl_superblocks, client_link) {
 		res = nfs_delegation_find_inode_server(server, fhandle);
-		if (res != NULL)
-			break;
+		if (res != ERR_PTR(-ENOENT))
+			return res;
 	}
 	rcu_read_unlock();
-	return res;
+	return ERR_PTR(-ENOENT);
 }
 
 static void nfs_delegation_mark_reclaim_server(struct nfs_server *server)