[PATCH 4.19 40/42] rxrpc: Fix lockup due to no error backoff after ack transmit error

Greg Kroah-Hartman
2018-11-21T19:08:00Z

From: David Howells <dhowells@redhat.com>

[ Upstream commit c7e86acfcee30794dc99a0759924bf7b9d43f1ca ]

If the network becomes (partially) unavailable, say by disabling IPv6, the
background ACK transmission routine can get itself into a tizzy by
proposing immediate ACK retransmission. Since we're in the call event
processor, that happens immediately without returning to the workqueue
manager.

The condition should clear after a while when either the network comes back
or the call times out.

Fix this by:

(1) When re-proposing an ACK on failed Tx, don't schedule it immediately.
This will allow a certain amount of time to elapse before we try
again.

(2) Enforce a return to the workqueue manager after a certain number of
iterations of the call processing loop.

(3) Add a backoff delay that increases the delay on deferred ACKs by a
jiffy per failed transmission to a limit of HZ. The backoff delay is
cleared on a successful return from kernel_sendmsg().

(4) Cancel calls immediately if the opening sendmsg fails. The layer
above can arrange retransmission or rotate to another server.

Fixes: 248f219cb8bc ("rxrpc: Rewrite the data and ack handling code")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/rxrpc/ar-internal.h | 1 +
net/rxrpc/call_event.c | 18 ++++++++++++++----
net/rxrpc/output.c | 35 +++++++++++++++++++++++++++++++----
3 files changed, 46 insertions(+), 8 deletions(-)

--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -611,6 +611,7 @@ struct rxrpc_call {
* not hard-ACK'd packet follows this.
*/
rxrpc_seq_t tx_top; /* Highest Tx slot allocated. */
+ u16 tx_backoff; /* Delay to insert due to Tx failure */

/* TCP-style slow-start congestion control [RFC5681]. Since the SMSS
* is fixed, we keep these numbers in terms of segments (ie. DATA
--- a/net/rxrpc/call_event.c
+++ b/net/rxrpc/call_event.c
@@ -123,6 +123,7 @@ static void __rxrpc_propose_ACK(struct r
else
ack_at = expiry;

+ ack_at += READ_ONCE(call->tx_backoff);
ack_at += now;
if (time_before(ack_at, call->ack_at)) {
WRITE_ONCE(call->ack_at, ack_at);
@@ -311,6 +312,7 @@ void rxrpc_process_call(struct work_stru
container_of(work, struct rxrpc_call, processor);
rxrpc_serial_t *send_ack;
unsigned long now, next, t;
+ unsigned int iterations = 0;

rxrpc_see_call(call);

@@ -319,6 +321,11 @@ void rxrpc_process_call(struct work_stru
call->debug_id, rxrpc_call_states[call->state], call->events);

recheck_state:
+ /* Limit the number of times we do this before returning to the manager */
+ iterations++;
+ if (iterations > 5)
+ goto requeue;
+
if (test_and_clear_bit(RXRPC_CALL_EV_ABORT, &call->events)) {
rxrpc_send_abort_packet(call);
goto recheck_state;
@@ -447,13 +454,16 @@ recheck_state:
rxrpc_reduce_call_timer(call, next, now, rxrpc_timer_restart);

/* other events may have been raised since we started checking */
- if (call->events && call->state < RXRPC_CALL_COMPLETE) {
- __rxrpc_queue_call(call);
- goto out;
- }
+ if (call->events && call->state < RXRPC_CALL_COMPLETE)
+ goto requeue;

out_put:
rxrpc_put_call(call, rxrpc_call_put);
out:
_leave("");
+ return;
+
+requeue:
+ __rxrpc_queue_call(call);
+ goto out;
}
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -35,6 +35,21 @@ struct rxrpc_abort_buffer {
static const char rxrpc_keepalive_string[] = "";

/*
+ * Increase Tx backoff on transmission failure and clear it on success.
+ */
+static void rxrpc_tx_backoff(struct rxrpc_call *call, int ret)
+{
+ if (ret < 0) {
+ u16 tx_backoff = READ_ONCE(call->tx_backoff);
+
+ if (tx_backoff < HZ)
+ WRITE_ONCE(call->tx_backoff, tx_backoff + 1);
+ } else {
+ WRITE_ONCE(call->tx_backoff, 0);
+ }
+}
+
+/*
* Arrange for a keepalive ping a certain time after we last transmitted. This
* lets the far side know we're still interested in this call and helps keep
* the route through any intervening firewall open.
@@ -210,6 +225,7 @@ int rxrpc_send_ack_packet(struct rxrpc_c
else
trace_rxrpc_tx_packet(call->debug_id, &pkt->whdr,
rxrpc_tx_point_call_ack);
+ rxrpc_tx_backoff(call, ret);

if (call->state < RXRPC_CALL_COMPLETE) {
if (ret < 0) {
@@ -218,7 +234,7 @@ int rxrpc_send_ack_packet(struct rxrpc_c
rxrpc_propose_ACK(call, pkt->ack.reason,
ntohs(pkt->ack.maxSkew),
ntohl(pkt->ack.serial),
- true, true,
+ false, true,
rxrpc_propose_ack_retry_tx);
} else {
spin_lock_bh(&call->lock);
@@ -300,7 +316,7 @@ int rxrpc_send_abort_packet(struct rxrpc
else
trace_rxrpc_tx_packet(call->debug_id, &pkt.whdr,
rxrpc_tx_point_call_abort);
-
+ rxrpc_tx_backoff(call, ret);

rxrpc_put_connection(conn);
return ret;
@@ -411,6 +427,7 @@ int rxrpc_send_data_packet(struct rxrpc_
else
trace_rxrpc_tx_packet(call->debug_id, &whdr,
rxrpc_tx_point_call_data_nofrag);
+ rxrpc_tx_backoff(call, ret);
if (ret == -EMSGSIZE)
goto send_fragmentable;

@@ -445,9 +462,18 @@ done:
rxrpc_reduce_call_timer(call, expect_rx_by, nowj,
rxrpc_timer_set_for_normal);
}
- }

- rxrpc_set_keepalive(call);
+ rxrpc_set_keepalive(call);
+ } else {
+ /* Cancel the call if the initial transmission fails,
+ * particularly if that's due to network routing issues that
+ * aren't going away anytime soon. The layer above can arrange
+ * the retransmission.
+ */
+ if (!test_and_set_bit(RXRPC_CALL_BEGAN_RX_TIMER, &call->flags))
+ rxrpc_set_call_completion(call, RXRPC_CALL_LOCAL_ERROR,
+ RX_USER_ABORT, ret);
+ }

_leave(" = %d [%u]", ret, call->peer->maxdata);
return ret;
@@ -506,6 +532,7 @@ send_fragmentable:
else
trace_rxrpc_tx_packet(call->debug_id, &whdr,
rxrpc_tx_point_call_data_frag);
+ rxrpc_tx_backoff(call, ret);

up_write(&conn->params.local->defrag_sem);
goto done;