NFSv4.1: Fix the callback 'highest_used_slotid' behaviour

Currently, there is no guarantee that we will call nfs4_cb_take_slot() even
though nfs4_callback_compound() will consistently call
nfs4_cb_free_slot() provided the cb_process_state has set the 'clp' field.
The result is that we can trigger the BUG_ON() upon the next call to
nfs4_cb_take_slot().

This patch fixes the above problem by using the slot id that was taken in
the CB_SEQUENCE operation as a flag for whether or not we need to call
nfs4_cb_free_slot().
It also fixes an atomicity problem: we need to set tbl->highest_used_slotid
atomically with the check for NFS4_SESSION_DRAINING, otherwise we end up
racing with the various tests in nfs4_begin_drain_session().

Cc: stable@kernel.org [2.6.38+]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/callback.h

@@ -38,6 +38,7 @@ enum nfs4_callback_opnum {
 struct cb_process_state {
 	__be32			drc_status;
 	struct nfs_client	*clp;
+	int			slotid;
 };
 
 struct cb_compound_hdr_arg {
@@ -166,7 +167,6 @@ extern unsigned nfs4_callback_layoutrecall(
 	void *dummy, struct cb_process_state *cps);
 
 extern void nfs4_check_drain_bc_complete(struct nfs4_session *ses);
-extern void nfs4_cb_take_slot(struct nfs_client *clp);
 
 struct cb_devicenotifyitem {
 	uint32_t		cbd_notify_type;

fs/nfs/callback_proc.c

@@ -348,7 +348,7 @@ validate_seqid(struct nfs4_slot_table *tbl, struct cb_sequenceargs * args)
 	/* Normal */
 	if (likely(args->csa_sequenceid == slot->seq_nr + 1)) {
 		slot->seq_nr++;
-		return htonl(NFS4_OK);
+		goto out_ok;
 	}
 
 	/* Replay */
@@ -367,11 +367,14 @@ validate_seqid(struct nfs4_slot_table *tbl, struct cb_sequenceargs * args)
 	/* Wraparound */
 	if (args->csa_sequenceid == 1 && (slot->seq_nr + 1) == 0) {
 		slot->seq_nr = 1;
-		return htonl(NFS4_OK);
+		goto out_ok;
 	}
 
 	/* Misordered request */
 	return htonl(NFS4ERR_SEQ_MISORDERED);
+out_ok:
+	tbl->highest_used_slotid = args->csa_slotid;
+	return htonl(NFS4_OK);
 }
 
 /*
@@ -433,26 +436,32 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
 			      struct cb_sequenceres *res,
 			      struct cb_process_state *cps)
 {
+	struct nfs4_slot_table *tbl;
 	struct nfs_client *clp;
 	int i;
 	__be32 status = htonl(NFS4ERR_BADSESSION);
 
-	cps->clp = NULL;
-
 	clp = nfs4_find_client_sessionid(args->csa_addr, &args->csa_sessionid);
 	if (clp == NULL)
 		goto out;
 
+	tbl = &clp->cl_session->bc_slot_table;
+
+	spin_lock(&tbl->slot_tbl_lock);
 	/* state manager is resetting the session */
 	if (test_bit(NFS4_SESSION_DRAINING, &clp->cl_session->session_state)) {
-		status = NFS4ERR_DELAY;
+		spin_unlock(&tbl->slot_tbl_lock);
+		status = htonl(NFS4ERR_DELAY);
 		goto out;
 	}
 
 	status = validate_seqid(&clp->cl_session->bc_slot_table, args);
+	spin_unlock(&tbl->slot_tbl_lock);
 	if (status)
 		goto out;
 
+	cps->slotid = args->csa_slotid;
+
 	/*
 	 * Check for pending referring calls.  If a match is found, a
 	 * related callback was received before the response to the original
@@ -469,7 +478,6 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
 	res->csr_slotid = args->csa_slotid;
 	res->csr_highestslotid = NFS41_BC_MAX_CALLBACKS - 1;
 	res->csr_target_highestslotid = NFS41_BC_MAX_CALLBACKS - 1;
-	nfs4_cb_take_slot(clp);
 
 out:
 	cps->clp = clp; /* put in nfs4_callback_compound */

fs/nfs/callback_xdr.c

@@ -754,26 +754,15 @@ static void nfs4_callback_free_slot(struct nfs4_session *session)
 	 * Let the state manager know callback processing done.
 	 * A single slot, so highest used slotid is either 0 or -1
 	 */
-	tbl->highest_used_slotid--;
+	tbl->highest_used_slotid = -1;
 	nfs4_check_drain_bc_complete(session);
 	spin_unlock(&tbl->slot_tbl_lock);
 }
 
-static void nfs4_cb_free_slot(struct nfs_client *clp)
+static void nfs4_cb_free_slot(struct cb_process_state *cps)
 {
-	if (clp && clp->cl_session)
-		nfs4_callback_free_slot(clp->cl_session);
-}
-
-/* A single slot, so highest used slotid is either 0 or -1 */
-void nfs4_cb_take_slot(struct nfs_client *clp)
-{
-	struct nfs4_slot_table *tbl = &clp->cl_session->bc_slot_table;
-
-	spin_lock(&tbl->slot_tbl_lock);
-	tbl->highest_used_slotid++;
-	BUG_ON(tbl->highest_used_slotid != 0);
-	spin_unlock(&tbl->slot_tbl_lock);
+	if (cps->slotid != -1)
+		nfs4_callback_free_slot(cps->clp->cl_session);
 }
 
 #else /* CONFIG_NFS_V4_1 */
@@ -784,7 +773,7 @@ preprocess_nfs41_op(int nop, unsigned int op_nr, struct callback_op **op)
 	return htonl(NFS4ERR_MINOR_VERS_MISMATCH);
 }
 
-static void nfs4_cb_free_slot(struct nfs_client *clp)
+static void nfs4_cb_free_slot(struct cb_process_state *cps)
 {
 }
 #endif /* CONFIG_NFS_V4_1 */
@@ -866,6 +855,7 @@ static __be32 nfs4_callback_compound(struct svc_rqst *rqstp, void *argp, void *r
 	struct cb_process_state cps = {
 		.drc_status = 0,
 		.clp = NULL,
+		.slotid = -1,
 	};
 	unsigned int nops = 0;
 
@@ -906,7 +896,7 @@ static __be32 nfs4_callback_compound(struct svc_rqst *rqstp, void *argp, void *r
 
 	*hdr_res.status = status;
 	*hdr_res.nops = htonl(nops);
-	nfs4_cb_free_slot(cps.clp);
+	nfs4_cb_free_slot(&cps);
 	nfs_put_client(cps.clp);
 	dprintk("%s: done, status = %u\n", __func__, ntohl(status));
 	return rpc_success;