NFSv4.1: Avoid false retries when RPC calls are interrupted

A 'false retry' in NFSv4.1 occurs when the client attempts to transmit a
new RPC call using a slot+sequence number combination that references an
already cached one. Currently, the Linux NFS client will do this if a
user process interrupts an RPC call that is in progress.
The problem with doing so is that we defeat the main mechanism used by
the server to differentiate between a new call and a replayed one. Even
if the server is able to perfectly cache the arguments of the old call,
it cannot know if the client intended to replay or send a new call.

The obvious fix is to bump the sequence number pre-emptively if an
RPC call is interrupted, but in order to deal with the corner cases
where the interrupted call is not actually received and processed by
the server, we need to interpret the error NFS4ERR_SEQ_MISORDERED
as a sign that we need to either wait or locate a correct sequence
number that lies between the value we sent, and the last value that
was acked by a SEQUENCE call on that slot.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Tested-by: Jason Tibbitts <tibbs@math.uh.edu>

fs/nfs/nfs4proc.c

@@ -730,13 +730,25 @@ static void nfs41_sequence_free_slot(struct nfs4_sequence_res *res)
 	res->sr_slot = NULL;
 }
 
+static void nfs4_slot_sequence_record_sent(struct nfs4_slot *slot,
+		u32 seqnr)
+{
+	if ((s32)(seqnr - slot->seq_nr_highest_sent) > 0)
+		slot->seq_nr_highest_sent = seqnr;
+}
+static void nfs4_slot_sequence_acked(struct nfs4_slot *slot,
+		u32 seqnr)
+{
+	slot->seq_nr_highest_sent = seqnr;
+	slot->seq_nr_last_acked = seqnr;
+}
+
 static int nfs41_sequence_process(struct rpc_task *task,
 		struct nfs4_sequence_res *res)
 {
 	struct nfs4_session *session;
 	struct nfs4_slot *slot = res->sr_slot;
 	struct nfs_client *clp;
-	bool interrupted = false;
 	int ret = 1;
 
 	if (slot == NULL)
@@ -747,16 +759,12 @@ static int nfs41_sequence_process(struct rpc_task *task,
 
 	session = slot->table->session;
 
-	if (slot->interrupted) {
-		if (res->sr_status != -NFS4ERR_DELAY)
-			slot->interrupted = 0;
-		interrupted = true;
-	}
-
 	trace_nfs4_sequence_done(session, res);
 	/* Check the SEQUENCE operation status */
 	switch (res->sr_status) {
 	case 0:
+		/* Mark this sequence number as having been acked */
+		nfs4_slot_sequence_acked(slot, slot->seq_nr);
 		/* Update the slot's sequence and clientid lease timer */
 		slot->seq_done = 1;
 		clp = session->clp;
@@ -771,9 +779,9 @@ static int nfs41_sequence_process(struct rpc_task *task,
 		 * sr_status remains 1 if an RPC level error occurred.
 		 * The server may or may not have processed the sequence
 		 * operation..
-		 * Mark the slot as having hosted an interrupted RPC call.
 		 */
-		slot->interrupted = 1;
+		nfs4_slot_sequence_record_sent(slot, slot->seq_nr);
+		slot->seq_done = 1;
 		goto out;
 	case -NFS4ERR_DELAY:
 		/* The server detected a resend of the RPC call and
@@ -784,6 +792,7 @@ static int nfs41_sequence_process(struct rpc_task *task,
 			__func__,
 			slot->slot_nr,
 			slot->seq_nr);
+		nfs4_slot_sequence_acked(slot, slot->seq_nr);
 		goto out_retry;
 	case -NFS4ERR_RETRY_UNCACHED_REP:
 	case -NFS4ERR_SEQ_FALSE_RETRY:
@@ -791,6 +800,7 @@ static int nfs41_sequence_process(struct rpc_task *task,
 		 * The server thinks we tried to replay a request.
 		 * Retry the call after bumping the sequence ID.
 		 */
+		nfs4_slot_sequence_acked(slot, slot->seq_nr);
 		goto retry_new_seq;
 	case -NFS4ERR_BADSLOT:
 		/*
@@ -801,21 +811,28 @@ static int nfs41_sequence_process(struct rpc_task *task,
 			goto session_recover;
 		goto retry_nowait;
 	case -NFS4ERR_SEQ_MISORDERED:
+		nfs4_slot_sequence_record_sent(slot, slot->seq_nr);
 		/*
-		 * Was the last operation on this sequence interrupted?
-		 * If so, retry after bumping the sequence number.
+		 * Were one or more calls using this slot interrupted?
+		 * If the server never received the request, then our
+		 * transmitted slot sequence number may be too high.
 		 */
-		if (interrupted)
-			goto retry_new_seq;
-		/*
-		 * Could this slot have been previously retired?
-		 * If so, then the server may be expecting seq_nr = 1!
-		 */
-		if (slot->seq_nr != 1) {
-			slot->seq_nr = 1;
+		if ((s32)(slot->seq_nr - slot->seq_nr_last_acked) > 1) {
+			slot->seq_nr--;
 			goto retry_nowait;
 		}
-		goto session_recover;
+		/*
+		 * RFC5661:
+		 * A retry might be sent while the original request is
+		 * still in progress on the replier. The replier SHOULD
+		 * deal with the issue by returning NFS4ERR_DELAY as the
+		 * reply to SEQUENCE or CB_SEQUENCE operation, but
+		 * implementations MAY return NFS4ERR_SEQ_MISORDERED.
+		 *
+		 * Restart the search after a delay.
+		 */
+		slot->seq_nr = slot->seq_nr_highest_sent;
+		goto out_retry;
 	default:
 		/* Just update the slot sequence no. */
 		slot->seq_done = 1;
@@ -906,17 +923,6 @@ static const struct rpc_call_ops nfs41_call_sync_ops = {
 	.rpc_call_done = nfs41_call_sync_done,
 };
 
-static void
-nfs4_sequence_process_interrupted(struct nfs_client *client,
-		struct nfs4_slot *slot, const struct cred *cred)
-{
-	struct rpc_task *task;
-
-	task = _nfs41_proc_sequence(client, cred, slot, true);
-	if (!IS_ERR(task))
-		rpc_put_task_async(task);
-}
-
 #else	/* !CONFIG_NFS_V4_1 */
 
 static int nfs4_sequence_process(struct rpc_task *task, struct nfs4_sequence_res *res)
@@ -937,14 +943,6 @@ int nfs4_sequence_done(struct rpc_task *task,
 }
 EXPORT_SYMBOL_GPL(nfs4_sequence_done);
 
-static void
-nfs4_sequence_process_interrupted(struct nfs_client *client,
-		struct nfs4_slot *slot, const struct cred *cred)
-{
-	WARN_ON_ONCE(1);
-	slot->interrupted = 0;
-}
-
 #endif	/* !CONFIG_NFS_V4_1 */
 
 static
@@ -982,7 +980,6 @@ int nfs4_setup_sequence(struct nfs_client *client,
 		task->tk_timeout = 0;
 	}
 
-	for (;;) {
 	spin_lock(&tbl->slot_tbl_lock);
 	/* The state manager will wait until the slot table is empty */
 	if (nfs4_slot_tbl_draining(tbl) && !args->sa_privileged)
@@ -997,12 +994,6 @@ int nfs4_setup_sequence(struct nfs_client *client,
 	}
 	spin_unlock(&tbl->slot_tbl_lock);
 
-		if (likely(!slot->interrupted))
-			break;
-		nfs4_sequence_process_interrupted(client,
-				slot, task->tk_msg.rpc_cred);
-	}
-
 	nfs4_sequence_attach_slot(args, res, slot);
 
 	trace_nfs4_setup_sequence(session, args);

fs/nfs/nfs4session.c

@@ -110,6 +110,8 @@ static struct nfs4_slot *nfs4_new_slot(struct nfs4_slot_table  *tbl,
 		slot->table = tbl;
 		slot->slot_nr = slotid;
 		slot->seq_nr = seq_init;
+		slot->seq_nr_highest_sent = seq_init;
+		slot->seq_nr_last_acked = seq_init - 1;
 	}
 	return slot;
 }
@@ -276,7 +278,8 @@ static void nfs4_reset_slot_table(struct nfs4_slot_table *tbl,
 	p = &tbl->slots;
 	while (*p) {
 		(*p)->seq_nr = ivalue;
-		(*p)->interrupted = 0;
+		(*p)->seq_nr_highest_sent = ivalue;
+		(*p)->seq_nr_last_acked = ivalue - 1;
 		p = &(*p)->next;
 	}
 	tbl->highest_used_slotid = NFS4_NO_SLOT;

fs/nfs/nfs4session.h

@@ -23,8 +23,9 @@ struct nfs4_slot {
 	unsigned long		generation;
 	u32			slot_nr;
 	u32		 	seq_nr;
-	unsigned int		interrupted : 1,
-				privileged : 1,
+	u32		 	seq_nr_last_acked;
+	u32		 	seq_nr_highest_sent;
+	unsigned int		privileged : 1,
 				seq_done : 1;
 };