/* Client connection-specific management code.
 *
 * Copyright (C) 2016 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 *
 *
 * Client connections need to be cached for a little while after they've made a
 * call so as to handle retransmitted DATA packets in case the server didn't
 * receive the final ACK or terminating ABORT we sent it.
 *
 * Client connections can be in one of a number of cache states:
 *
 *  (1) INACTIVE - The connection is not held in any list and may not have been
 *      exposed to the world.  If it has been previously exposed, it was
 *      discarded from the idle list after expiring.
 *
 *  (2) WAITING - The connection is waiting for the number of client conns to
 *      drop below the maximum capacity.  Calls may be in progress upon it from
 *      when it was active and got culled.
 *
 *      The connection is on the rxrpc_waiting_client_conns list which is kept
 *      in to-be-granted order.  Culled conns with waiters go to the back of
 *      the queue just like new conns.
 *
 *  (3) ACTIVE - The connection has at least one call in progress upon it, it
 *      may freely grant available channels to new calls and calls may be
 *      waiting on it for channels to become available.
 *
 *      The connection is on the rxrpc_active_client_conns list which is kept
 *      in activation order for culling purposes.
 *
 *      rxrpc_nr_active_client_conns is held incremented also.
 *
 *  (4) CULLED - The connection got summarily culled to try and free up
 *      capacity.  Calls currently in progress on the connection are allowed to
 *      continue, but new calls will have to wait.  There can be no waiters in
 *      this state - the conn would have to go to the WAITING state instead.
 *
 *  (5) IDLE - The connection has no calls in progress upon it and must have
 *      been exposed to the world (ie. the EXPOSED flag must be set).  When it
 *      expires, the EXPOSED flag is cleared and the connection transitions to
 *      the INACTIVE state.
 *
 *      The connection is on the rxrpc_idle_client_conns list which is kept in
 *      order of how soon they'll expire.
 *
 * There are flags of relevance to the cache:
 *
 *  (1) EXPOSED - The connection ID got exposed to the world.  If this flag is
 *      set, an extra ref is added to the connection preventing it from being
 *      reaped when it has no calls outstanding.  This flag is cleared and the
 *      ref dropped when a conn is discarded from the idle list.
 *
 *      This allows us to move terminal call state retransmission to the
 *      connection and to discard the call immediately we think it is done
 *      with.  It also give us a chance to reuse the connection.
 *
 *  (2) DONT_REUSE - The connection should be discarded as soon as possible and
 *      should not be reused.  This is set when an exclusive connection is used
 *      or a call ID counter overflows.
 *
 * The caching state may only be changed if the cache lock is held.
 *
 * There are two idle client connection expiry durations.  If the total number
 * of connections is below the reap threshold, we use the normal duration; if
 * it's above, we use the fast duration.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/slab.h>
#include <linux/idr.h>
#include <linux/timer.h>
#include "ar-internal.h"

__read_mostly unsigned int rxrpc_max_client_connections = 1000;
__read_mostly unsigned int rxrpc_reap_client_connections = 900;
__read_mostly unsigned int rxrpc_conn_idle_client_expiry = 2 * 60 * HZ;
__read_mostly unsigned int rxrpc_conn_idle_client_fast_expiry = 2 * HZ;

static unsigned int rxrpc_nr_client_conns;
static unsigned int rxrpc_nr_active_client_conns;
static __read_mostly bool rxrpc_kill_all_client_conns;

static DEFINE_SPINLOCK(rxrpc_client_conn_cache_lock);
static DEFINE_SPINLOCK(rxrpc_client_conn_discard_mutex);
static LIST_HEAD(rxrpc_waiting_client_conns);
static LIST_HEAD(rxrpc_active_client_conns);
static LIST_HEAD(rxrpc_idle_client_conns);

/*
 * We use machine-unique IDs for our client connections.
 */
DEFINE_IDR(rxrpc_client_conn_ids);
static DEFINE_SPINLOCK(rxrpc_conn_id_lock);

static void rxrpc_cull_active_client_conns(void);
static void rxrpc_discard_expired_client_conns(struct work_struct *);

static DECLARE_DELAYED_WORK(rxrpc_client_conn_reap,
                            rxrpc_discard_expired_client_conns);

const char rxrpc_conn_cache_states[RXRPC_CONN__NR_CACHE_STATES][5] = {
        [RXRPC_CONN_CLIENT_INACTIVE]    = "Inac",
        [RXRPC_CONN_CLIENT_WAITING]     = "Wait",
        [RXRPC_CONN_CLIENT_ACTIVE]      = "Actv",
        [RXRPC_CONN_CLIENT_CULLED]      = "Cull",
        [RXRPC_CONN_CLIENT_IDLE]        = "Idle",
};

/*
 * Get a connection ID and epoch for a client connection from the global pool.
 * The connection struct pointer is then recorded in the idr radix tree.  The
 * epoch doesn't change until the client is rebooted (or, at least, unless the
 * module is unloaded).
 */
static int rxrpc_get_client_connection_id(struct rxrpc_connection *conn,
                                          gfp_t gfp)
{
        int id;

        _enter("");

        idr_preload(gfp);
        spin_lock(&rxrpc_conn_id_lock);

        id = idr_alloc_cyclic(&rxrpc_client_conn_ids, conn,
                              1, 0x40000000, GFP_NOWAIT);
        if (id < 0)
                goto error;

        spin_unlock(&rxrpc_conn_id_lock);
        idr_preload_end();

        conn->proto.epoch = rxrpc_epoch;
        conn->proto.cid = id << RXRPC_CIDSHIFT;
        set_bit(RXRPC_CONN_HAS_IDR, &conn->flags);
        _leave(" [CID %x]", conn->proto.cid);
        return 0;

error:
        spin_unlock(&rxrpc_conn_id_lock);
        idr_preload_end();
        _leave(" = %d", id);
        return id;
}

/*
 * Release a connection ID for a client connection from the global pool.
 */
static void rxrpc_put_client_connection_id(struct rxrpc_connection *conn)
{
        if (test_bit(RXRPC_CONN_HAS_IDR, &conn->flags)) {
                spin_lock(&rxrpc_conn_id_lock);
                idr_remove(&rxrpc_client_conn_ids,
                           conn->proto.cid >> RXRPC_CIDSHIFT);
                spin_unlock(&rxrpc_conn_id_lock);
        }
}

/*
 * Destroy the client connection ID tree.
 */
void rxrpc_destroy_client_conn_ids(void)
{
        struct rxrpc_connection *conn;
        int id;

        if (!idr_is_empty(&rxrpc_client_conn_ids)) {
                idr_for_each_entry(&rxrpc_client_conn_ids, conn, id) {
                        pr_err("AF_RXRPC: Leaked client conn %p {%d}\n",
                               conn, atomic_read(&conn->usage));
                }
                BUG();
        }

        idr_destroy(&rxrpc_client_conn_ids);
}

/*
 * Allocate a client connection.
 */
static struct rxrpc_connection *
rxrpc_alloc_client_connection(struct rxrpc_conn_parameters *cp, gfp_t gfp)
{
        struct rxrpc_connection *conn;
        int ret;

        _enter("");

        conn = rxrpc_alloc_connection(gfp);
        if (!conn) {
                _leave(" = -ENOMEM");
                return ERR_PTR(-ENOMEM);
        }

        atomic_set(&conn->usage, 1);
        if (cp->exclusive)
                __set_bit(RXRPC_CONN_DONT_REUSE, &conn->flags);

        conn->params            = *cp;
        conn->out_clientflag    = RXRPC_CLIENT_INITIATED;
        conn->state             = RXRPC_CONN_CLIENT;

        ret = rxrpc_get_client_connection_id(conn, gfp);
        if (ret < 0)
                goto error_0;

        ret = rxrpc_init_client_conn_security(conn);
        if (ret < 0)
                goto error_1;

        ret = conn->security->prime_packet_security(conn);
        if (ret < 0)
                goto error_2;

        write_lock(&rxrpc_connection_lock);
        list_add_tail(&conn->proc_link, &rxrpc_connection_proc_list);
        write_unlock(&rxrpc_connection_lock);

        /* We steal the caller's peer ref. */
        cp->peer = NULL;
        rxrpc_get_local(conn->params.local);
        key_get(conn->params.key);

        trace_rxrpc_conn(conn, rxrpc_conn_new_client, atomic_read(&conn->usage),
                         __builtin_return_address(0));
        trace_rxrpc_client(conn, -1, rxrpc_client_alloc);
        _leave(" = %p", conn);
        return conn;

error_2:
        conn->security->clear(conn);
error_1:
        rxrpc_put_client_connection_id(conn);
error_0:
        kfree(conn);
        _leave(" = %d", ret);
        return ERR_PTR(ret);
}

/*
 * Determine if a connection may be reused.
 */
static bool rxrpc_may_reuse_conn(struct rxrpc_connection *conn)
{
        int id_cursor, id, distance, limit;

        if (test_bit(RXRPC_CONN_DONT_REUSE, &conn->flags))
                goto dont_reuse;

        if (conn->proto.epoch != rxrpc_epoch)
                goto mark_dont_reuse;

        /* The IDR tree gets very expensive on memory if the connection IDs are
         * widely scattered throughout the number space, so we shall want to
         * kill off connections that, say, have an ID more than about four
         * times the maximum number of client conns away from the current
         * allocation point to try and keep the IDs concentrated.
         */
        id_cursor = READ_ONCE(rxrpc_client_conn_ids.cur);
        id = conn->proto.cid >> RXRPC_CIDSHIFT;
        distance = id - id_cursor;
        if (distance < 0)
                distance = -distance;
        limit = round_up(rxrpc_max_client_connections, IDR_SIZE) * 4;
        if (distance > limit)
                goto mark_dont_reuse;

        return true;

mark_dont_reuse:
        set_bit(RXRPC_CONN_DONT_REUSE, &conn->flags);
dont_reuse:
        return false;
}

/*
 * Create or find a client connection to use for a call.
 *
 * If we return with a connection, the call will be on its waiting list.  It's
 * left to the caller to assign a channel and wake up the call.
 */
static int rxrpc_get_client_conn(struct rxrpc_call *call,
                                 struct rxrpc_conn_parameters *cp,
                                 struct sockaddr_rxrpc *srx,
                                 gfp_t gfp)
{
        struct rxrpc_connection *conn, *candidate = NULL;
        struct rxrpc_local *local = cp->local;
        struct rb_node *p, **pp, *parent;
        long diff;
        int ret = -ENOMEM;

        _enter("{%d,%lx},", call->debug_id, call->user_call_ID);

        cp->peer = rxrpc_lookup_peer(cp->local, srx, gfp);
        if (!cp->peer)
                goto error;

        /* If the connection is not meant to be exclusive, search the available
         * connections to see if the connection we want to use already exists.
         */
        if (!cp->exclusive) {
                _debug("search 1");
                spin_lock(&local->client_conns_lock);
                p = local->client_conns.rb_node;
                while (p) {
                        conn = rb_entry(p, struct rxrpc_connection, client_node);

#define cmp(X) ((long)conn->params.X - (long)cp->X)
                        diff = (cmp(peer) ?:
                                cmp(key) ?:
                                cmp(security_level));
#undef cmp
                        if (diff < 0) {
                                p = p->rb_left;
                        } else if (diff > 0) {
                                p = p->rb_right;
                        } else {
                                if (rxrpc_may_reuse_conn(conn) &&
                                    rxrpc_get_connection_maybe(conn))
                                        goto found_extant_conn;
                                /* The connection needs replacing.  It's better
                                 * to effect that when we have something to
                                 * replace it with so that we don't have to
                                 * rebalance the tree twice.
                                 */
                                break;
                        }
                }
                spin_unlock(&local->client_conns_lock);
        }

        /* There wasn't a connection yet or we need an exclusive connection.
         * We need to create a candidate and then potentially redo the search
         * in case we're racing with another thread also trying to connect on a
         * shareable connection.
         */
        _debug("new conn");
        candidate = rxrpc_alloc_client_connection(cp, gfp);
        if (IS_ERR(candidate)) {
                ret = PTR_ERR(candidate);
                goto error_peer;
        }

        /* Add the call to the new connection's waiting list in case we're
         * going to have to wait for the connection to come live.  It's our
         * connection, so we want first dibs on the channel slots.  We would
         * normally have to take channel_lock but we do this before anyone else
         * can see the connection.
         */
        list_add_tail(&call->chan_wait_link, &candidate->waiting_calls);

        if (cp->exclusive) {
                call->conn = candidate;
                call->security_ix = candidate->security_ix;
                _leave(" = 0 [exclusive %d]", candidate->debug_id);
                return 0;
        }

        /* Publish the new connection for userspace to find.  We need to redo
         * the search before doing this lest we race with someone else adding a
         * conflicting instance.
         */
        _debug("search 2");
        spin_lock(&local->client_conns_lock);

        pp = &local->client_conns.rb_node;
        parent = NULL;
        while (*pp) {
                parent = *pp;
                conn = rb_entry(parent, struct rxrpc_connection, client_node);

#define cmp(X) ((long)conn->params.X - (long)candidate->params.X)
                diff = (cmp(peer) ?:
                        cmp(key) ?:
                        cmp(security_level));
#undef cmp
                if (diff < 0) {
                        pp = &(*pp)->rb_left;
                } else if (diff > 0) {
                        pp = &(*pp)->rb_right;
                } else {
                        if (rxrpc_may_reuse_conn(conn) &&
                            rxrpc_get_connection_maybe(conn))
                                goto found_extant_conn;
                        /* The old connection is from an outdated epoch. */
                        _debug("replace conn");
                        clear_bit(RXRPC_CONN_IN_CLIENT_CONNS, &conn->flags);
                        rb_replace_node(&conn->client_node,
                                        &candidate->client_node,
                                        &local->client_conns);
                        trace_rxrpc_client(conn, -1, rxrpc_client_replace);
                        goto candidate_published;
                }
        }

        _debug("new conn");
        rb_link_node(&candidate->client_node, parent, pp);
        rb_insert_color(&candidate->client_node, &local->client_conns);

candidate_published:
        set_bit(RXRPC_CONN_IN_CLIENT_CONNS, &candidate->flags);
        call->conn = candidate;
        call->security_ix = candidate->security_ix;
        spin_unlock(&local->client_conns_lock);
        _leave(" = 0 [new %d]", candidate->debug_id);
        return 0;

        /* We come here if we found a suitable connection already in existence.
         * Discard any candidate we may have allocated, and try to get a
         * channel on this one.
         */
found_extant_conn:
        _debug("found conn");
        spin_unlock(&local->client_conns_lock);

        if (candidate) {
                trace_rxrpc_client(candidate, -1, rxrpc_client_duplicate);
                rxrpc_put_connection(candidate);
                candidate = NULL;
        }

        spin_lock(&conn->channel_lock);
        call->conn = conn;
        call->security_ix = conn->security_ix;
        list_add(&call->chan_wait_link, &conn->waiting_calls);
        spin_unlock(&conn->channel_lock);
        _leave(" = 0 [extant %d]", conn->debug_id);
        return 0;

error_peer:
        rxrpc_put_peer(cp->peer);
        cp->peer = NULL;
error:
        _leave(" = %d", ret);
        return ret;
}

/*
 * Activate a connection.
 */
static void rxrpc_activate_conn(struct rxrpc_connection *conn)
{
        trace_rxrpc_client(conn, -1, rxrpc_client_to_active);
        conn->cache_state = RXRPC_CONN_CLIENT_ACTIVE;
        rxrpc_nr_active_client_conns++;
        list_move_tail(&conn->cache_link, &rxrpc_active_client_conns);
}

/*
 * Attempt to animate a connection for a new call.
 *
 * If it's not exclusive, the connection is in the endpoint tree, and we're in
 * the conn's list of those waiting to grab a channel.  There is, however, a
 * limit on the number of live connections allowed at any one time, so we may
 * have to wait for capacity to become available.
 *
 * Note that a connection on the waiting queue might *also* have active
 * channels if it has been culled to make space and then re-requested by a new
 * call.
 */
static void rxrpc_animate_client_conn(struct rxrpc_connection *conn)
{
        unsigned int nr_conns;

        _enter("%d,%d", conn->debug_id, conn->cache_state);

        if (conn->cache_state == RXRPC_CONN_CLIENT_ACTIVE)
                goto out;

        spin_lock(&rxrpc_client_conn_cache_lock);

        nr_conns = rxrpc_nr_client_conns;
        if (!test_and_set_bit(RXRPC_CONN_COUNTED, &conn->flags)) {
                trace_rxrpc_client(conn, -1, rxrpc_client_count);
                rxrpc_nr_client_conns = nr_conns + 1;
        }

        switch (conn->cache_state) {
        case RXRPC_CONN_CLIENT_ACTIVE:
        case RXRPC_CONN_CLIENT_WAITING:
                break;

        case RXRPC_CONN_CLIENT_INACTIVE:
        case RXRPC_CONN_CLIENT_CULLED:
        case RXRPC_CONN_CLIENT_IDLE:
                if (nr_conns >= rxrpc_max_client_connections)
                        goto wait_for_capacity;
                goto activate_conn;

        default:
                BUG();
        }

out_unlock:
        spin_unlock(&rxrpc_client_conn_cache_lock);
out:
        _leave(" [%d]", conn->cache_state);
        return;

activate_conn:
        _debug("activate");
        rxrpc_activate_conn(conn);
        goto out_unlock;

wait_for_capacity:
        _debug("wait");
        trace_rxrpc_client(conn, -1, rxrpc_client_to_waiting);
        conn->cache_state = RXRPC_CONN_CLIENT_WAITING;
        list_move_tail(&conn->cache_link, &rxrpc_waiting_client_conns);
        goto out_unlock;
}

/*
 * Deactivate a channel.
 */
static void rxrpc_deactivate_one_channel(struct rxrpc_connection *conn,
                                         unsigned int channel)
{
        struct rxrpc_channel *chan = &conn->channels[channel];

        rcu_assign_pointer(chan->call, NULL);
        conn->active_chans &= ~(1 << channel);
}

/*
 * Assign a channel to the call at the front of the queue and wake the call up.
 * We don't increment the callNumber counter until this number has been exposed
 * to the world.
 */
static void rxrpc_activate_one_channel(struct rxrpc_connection *conn,
                                       unsigned int channel)
{
        struct rxrpc_channel *chan = &conn->channels[channel];
        struct rxrpc_call *call = list_entry(conn->waiting_calls.next,
                                             struct rxrpc_call, chan_wait_link);
        u32 call_id = chan->call_counter + 1;

        trace_rxrpc_client(conn, channel, rxrpc_client_chan_activate);

        write_lock_bh(&call->state_lock);
        call->state = RXRPC_CALL_CLIENT_SEND_REQUEST;
        write_unlock_bh(&call->state_lock);

        rxrpc_see_call(call);
        list_del_init(&call->chan_wait_link);
        conn->active_chans |= 1 << channel;
        call->peer      = rxrpc_get_peer(conn->params.peer);
        call->cid       = conn->proto.cid | channel;
        call->call_id   = call_id;

        _net("CONNECT call %08x:%08x as call %d on conn %d",
             call->cid, call->call_id, call->debug_id, conn->debug_id);

        /* Paired with the read barrier in rxrpc_wait_for_channel().  This
         * orders cid and epoch in the connection wrt to call_id without the
         * need to take the channel_lock.
         *
         * We provisionally assign a callNumber at this point, but we don't
         * confirm it until the call is about to be exposed.
         *
         * TODO: Pair with a barrier in the data_ready handler when that looks
         * at the call ID through a connection channel.
         */
        smp_wmb();
        chan->call_id   = call_id;
        rcu_assign_pointer(chan->call, call);
        wake_up(&call->waitq);
}

/*
 * Assign channels and callNumbers to waiting calls with channel_lock
 * held by caller.
 */
static void rxrpc_activate_channels_locked(struct rxrpc_connection *conn)
{
        u8 avail, mask;

        switch (conn->cache_state) {
        case RXRPC_CONN_CLIENT_ACTIVE:
                mask = RXRPC_ACTIVE_CHANS_MASK;
                break;
        default:
                return;
        }

        while (!list_empty(&conn->waiting_calls) &&
               (avail = ~conn->active_chans,
                avail &= mask,
                avail != 0))
                rxrpc_activate_one_channel(conn, __ffs(avail));
}

/*
 * Assign channels and callNumbers to waiting calls.
 */
static void rxrpc_activate_channels(struct rxrpc_connection *conn)
{
        _enter("%d", conn->debug_id);

        trace_rxrpc_client(conn, -1, rxrpc_client_activate_chans);

        if (conn->active_chans == RXRPC_ACTIVE_CHANS_MASK)
                return;

        spin_lock(&conn->channel_lock);
        rxrpc_activate_channels_locked(conn);
        spin_unlock(&conn->channel_lock);
        _leave("");
}

/*
 * Wait for a callNumber and a channel to be granted to a call.
 */
static int rxrpc_wait_for_channel(struct rxrpc_call *call, gfp_t gfp)
{
        int ret = 0;

        _enter("%d", call->debug_id);

        if (!call->call_id) {
                DECLARE_WAITQUEUE(myself, current);

                if (!gfpflags_allow_blocking(gfp)) {
                        ret = -EAGAIN;
                        goto out;
                }

                add_wait_queue_exclusive(&call->waitq, &myself);
                for (;;) {
                        set_current_state(TASK_INTERRUPTIBLE);
                        if (call->call_id)
                                break;
                        if (signal_pending(current)) {
                                ret = -ERESTARTSYS;
                                break;
                        }
                        schedule();
                }
                remove_wait_queue(&call->waitq, &myself);
                __set_current_state(TASK_RUNNING);
        }

        /* Paired with the write barrier in rxrpc_activate_one_channel(). */
        smp_rmb();

out:
        _leave(" = %d", ret);
        return ret;
}

/*
 * find a connection for a call
 * - called in process context with IRQs enabled
 */
int rxrpc_connect_call(struct rxrpc_call *call,
                       struct rxrpc_conn_parameters *cp,
                       struct sockaddr_rxrpc *srx,
                       gfp_t gfp)
{
        int ret;

        _enter("{%d,%lx},", call->debug_id, call->user_call_ID);

        rxrpc_discard_expired_client_conns(NULL);
        rxrpc_cull_active_client_conns();

        ret = rxrpc_get_client_conn(call, cp, srx, gfp);
        if (ret < 0)
                return ret;

        rxrpc_animate_client_conn(call->conn);
        rxrpc_activate_channels(call->conn);

        ret = rxrpc_wait_for_channel(call, gfp);
        if (ret < 0)
                rxrpc_disconnect_client_call(call);

        _leave(" = %d", ret);
        return ret;
}

/*
 * Note that a connection is about to be exposed to the world.  Once it is
 * exposed, we maintain an extra ref on it that stops it from being summarily
 * discarded before it's (a) had a chance to deal with retransmission and (b)
 * had a chance at re-use (the per-connection security negotiation is
 * expensive).
 */
static void rxrpc_expose_client_conn(struct rxrpc_connection *conn,
                                     unsigned int channel)
{
        if (!test_and_set_bit(RXRPC_CONN_EXPOSED, &conn->flags)) {
                trace_rxrpc_client(conn, channel, rxrpc_client_exposed);
                rxrpc_get_connection(conn);
        }
}

/*
 * Note that a call, and thus a connection, is about to be exposed to the
 * world.
 */
void rxrpc_expose_client_call(struct rxrpc_call *call)
{
        unsigned int channel = call->cid & RXRPC_CHANNELMASK;
        struct rxrpc_connection *conn = call->conn;
        struct rxrpc_channel *chan = &conn->channels[channel];

        if (!test_and_set_bit(RXRPC_CALL_EXPOSED, &call->flags)) {
                /* Mark the call ID as being used.  If the callNumber counter
                 * exceeds ~2 billion, we kill the connection after its
                 * outstanding calls have finished so that the counter doesn't
                 * wrap.
                 */
                chan->call_counter++;
                if (chan->call_counter >= INT_MAX)
                        set_bit(RXRPC_CONN_DONT_REUSE, &conn->flags);
                rxrpc_expose_client_conn(conn, channel);
        }
}

/*
 * Disconnect a client call.
 */
void rxrpc_disconnect_client_call(struct rxrpc_call *call)
{
        unsigned int channel = call->cid & RXRPC_CHANNELMASK;
        struct rxrpc_connection *conn = call->conn;
        struct rxrpc_channel *chan = &conn->channels[channel];

        trace_rxrpc_client(conn, channel, rxrpc_client_chan_disconnect);
        call->conn = NULL;

        spin_lock(&conn->channel_lock);

        /* Calls that have never actually been assigned a channel can simply be
         * discarded.  If the conn didn't get used either, it will follow
         * immediately unless someone else grabs it in the meantime.
         */
        if (!list_empty(&call->chan_wait_link)) {
                _debug("call is waiting");
                ASSERTCMP(call->call_id, ==, 0);
                ASSERT(!test_bit(RXRPC_CALL_EXPOSED, &call->flags));
                list_del_init(&call->chan_wait_link);

                trace_rxrpc_client(conn, channel, rxrpc_client_chan_unstarted);

                /* We must deactivate or idle the connection if it's now
                 * waiting for nothing.
                 */
                spin_lock(&rxrpc_client_conn_cache_lock);
                if (conn->cache_state == RXRPC_CONN_CLIENT_WAITING &&
                    list_empty(&conn->waiting_calls) &&
                    !conn->active_chans)
                        goto idle_connection;
                goto out;
        }

        ASSERTCMP(rcu_access_pointer(chan->call), ==, call);

        /* If a client call was exposed to the world, we save the result for
         * retransmission.
         *
         * We use a barrier here so that the call number and abort code can be
         * read without needing to take a lock.
         *
         * TODO: Make the incoming packet handler check this and handle
         * terminal retransmission without requiring access to the call.
         */
        if (test_bit(RXRPC_CALL_EXPOSED, &call->flags)) {
                _debug("exposed %u,%u", call->call_id, call->abort_code);
                __rxrpc_disconnect_call(conn, call);
        }

        /* See if we can pass the channel directly to another call. */
        if (conn->cache_state == RXRPC_CONN_CLIENT_ACTIVE &&
            !list_empty(&conn->waiting_calls)) {
                trace_rxrpc_client(conn, channel, rxrpc_client_chan_pass);
                rxrpc_activate_one_channel(conn, channel);
                goto out_2;
        }

        /* Things are more complex and we need the cache lock.  We might be
         * able to simply idle the conn or it might now be lurking on the wait
         * list.  It might even get moved back to the active list whilst we're
         * waiting for the lock.
         */
        spin_lock(&rxrpc_client_conn_cache_lock);

        switch (conn->cache_state) {
        case RXRPC_CONN_CLIENT_ACTIVE:
                if (list_empty(&conn->waiting_calls)) {
                        rxrpc_deactivate_one_channel(conn, channel);
                        if (!conn->active_chans) {
                                rxrpc_nr_active_client_conns--;
                                goto idle_connection;
                        }
                        goto out;
                }

                trace_rxrpc_client(conn, channel, rxrpc_client_chan_pass);
                rxrpc_activate_one_channel(conn, channel);
                goto out;

        case RXRPC_CONN_CLIENT_CULLED:
                rxrpc_deactivate_one_channel(conn, channel);
                ASSERT(list_empty(&conn->waiting_calls));
                if (!conn->active_chans)
                        goto idle_connection;
                goto out;

        case RXRPC_CONN_CLIENT_WAITING:
                rxrpc_deactivate_one_channel(conn, channel);
                goto out;

        default:
                BUG();
        }

out:
        spin_unlock(&rxrpc_client_conn_cache_lock);
out_2:
        spin_unlock(&conn->channel_lock);
        rxrpc_put_connection(conn);
        _leave("");
        return;

idle_connection:
        /* As no channels remain active, the connection gets deactivated
         * immediately or moved to the idle list for a short while.
         */
        if (test_bit(RXRPC_CONN_EXPOSED, &conn->flags)) {
                trace_rxrpc_client(conn, channel, rxrpc_client_to_idle);
                conn->idle_timestamp = jiffies;
                conn->cache_state = RXRPC_CONN_CLIENT_IDLE;
                list_move_tail(&conn->cache_link, &rxrpc_idle_client_conns);
                if (rxrpc_idle_client_conns.next == &conn->cache_link &&
                    !rxrpc_kill_all_client_conns)
                        queue_delayed_work(rxrpc_workqueue,
                                           &rxrpc_client_conn_reap,
                                           rxrpc_conn_idle_client_expiry);
        } else {
                trace_rxrpc_client(conn, channel, rxrpc_client_to_inactive);
                conn->cache_state = RXRPC_CONN_CLIENT_INACTIVE;
                list_del_init(&conn->cache_link);
        }
        goto out;
}

/*
 * Clean up a dead client connection.
 */
static struct rxrpc_connection *
rxrpc_put_one_client_conn(struct rxrpc_connection *conn)
{
        struct rxrpc_connection *next = NULL;
        struct rxrpc_local *local = conn->params.local;
        unsigned int nr_conns;

        trace_rxrpc_client(conn, -1, rxrpc_client_cleanup);

        if (test_bit(RXRPC_CONN_IN_CLIENT_CONNS, &conn->flags)) {
                spin_lock(&local->client_conns_lock);
                if (test_and_clear_bit(RXRPC_CONN_IN_CLIENT_CONNS,
                                       &conn->flags))
                        rb_erase(&conn->client_node, &local->client_conns);
                spin_unlock(&local->client_conns_lock);
        }

        rxrpc_put_client_connection_id(conn);

        ASSERTCMP(conn->cache_state, ==, RXRPC_CONN_CLIENT_INACTIVE);

        if (test_bit(RXRPC_CONN_COUNTED, &conn->flags)) {
                trace_rxrpc_client(conn, -1, rxrpc_client_uncount);
                spin_lock(&rxrpc_client_conn_cache_lock);
                nr_conns = --rxrpc_nr_client_conns;

                if (nr_conns < rxrpc_max_client_connections &&
                    !list_empty(&rxrpc_waiting_client_conns)) {
                        next = list_entry(rxrpc_waiting_client_conns.next,
                                          struct rxrpc_connection, cache_link);
                        rxrpc_get_connection(next);
                        rxrpc_activate_conn(next);
                }

                spin_unlock(&rxrpc_client_conn_cache_lock);
        }

        rxrpc_kill_connection(conn);
        if (next)
                rxrpc_activate_channels(next);

        /* We need to get rid of the temporary ref we took upon next, but we
         * can't call rxrpc_put_connection() recursively.
         */
        return next;
}

/*
 * Clean up a dead client connections.
 */
void rxrpc_put_client_conn(struct rxrpc_connection *conn)
{
        const void *here = __builtin_return_address(0);
        int n;

        do {
                n = atomic_dec_return(&conn->usage);
                trace_rxrpc_conn(conn, rxrpc_conn_put_client, n, here);
                if (n > 0)
                        return;
                ASSERTCMP(n, >=, 0);

                conn = rxrpc_put_one_client_conn(conn);
        } while (conn);
}

/*
 * Kill the longest-active client connections to make room for new ones.
 */
static void rxrpc_cull_active_client_conns(void)
{
        struct rxrpc_connection *conn;
        unsigned int nr_conns = rxrpc_nr_client_conns;
        unsigned int nr_active, limit;

        _enter("");

        ASSERTCMP(nr_conns, >=, 0);
        if (nr_conns < rxrpc_max_client_connections) {
                _leave(" [ok]");
                return;
        }
        limit = rxrpc_reap_client_connections;

        spin_lock(&rxrpc_client_conn_cache_lock);
        nr_active = rxrpc_nr_active_client_conns;

        while (nr_active > limit) {
                ASSERT(!list_empty(&rxrpc_active_client_conns));
                conn = list_entry(rxrpc_active_client_conns.next,
                                  struct rxrpc_connection, cache_link);
                ASSERTCMP(conn->cache_state, ==, RXRPC_CONN_CLIENT_ACTIVE);

                if (list_empty(&conn->waiting_calls)) {
                        trace_rxrpc_client(conn, -1, rxrpc_client_to_culled);
                        conn->cache_state = RXRPC_CONN_CLIENT_CULLED;
                        list_del_init(&conn->cache_link);
                } else {
                        trace_rxrpc_client(conn, -1, rxrpc_client_to_waiting);
                        conn->cache_state = RXRPC_CONN_CLIENT_WAITING;
                        list_move_tail(&conn->cache_link,
                                       &rxrpc_waiting_client_conns);
                }

                nr_active--;
        }

        rxrpc_nr_active_client_conns = nr_active;
        spin_unlock(&rxrpc_client_conn_cache_lock);
        ASSERTCMP(nr_active, >=, 0);
        _leave(" [culled]");
}

/*
 * Discard expired client connections from the idle list.  Each conn in the
 * idle list has been exposed and holds an extra ref because of that.
 *
 * This may be called from conn setup or from a work item so cannot be
 * considered non-reentrant.
 */
static void rxrpc_discard_expired_client_conns(struct work_struct *work)
{
        struct rxrpc_connection *conn;
        unsigned long expiry, conn_expires_at, now;
        unsigned int nr_conns;
        bool did_discard = false;

        _enter("%c", work ? 'w' : 'n');

        if (list_empty(&rxrpc_idle_client_conns)) {
                _leave(" [empty]");
                return;
        }

        /* Don't double up on the discarding */
        if (!spin_trylock(&rxrpc_client_conn_discard_mutex)) {
                _leave(" [already]");
                return;
        }

        /* We keep an estimate of what the number of conns ought to be after

         * we've discarded some so that we don't overdo the discarding.
         */
        nr_conns = rxrpc_nr_client_conns;

next:
        spin_lock(&rxrpc_client_conn_cache_lock);

        if (list_empty(&rxrpc_idle_client_conns))
                goto out;

        conn = list_entry(rxrpc_idle_client_conns.next,
                          struct rxrpc_connection, cache_link);
        ASSERT(test_bit(RXRPC_CONN_EXPOSED, &conn->flags));

        if (!rxrpc_kill_all_client_conns) {
                /* If the number of connections is over the reap limit, we
                 * expedite discard by reducing the expiry timeout.  We must,
                 * however, have at least a short grace period to be able to do
                 * final-ACK or ABORT retransmission.
                 */
                expiry = rxrpc_conn_idle_client_expiry;
                if (nr_conns > rxrpc_reap_client_connections)
                        expiry = rxrpc_conn_idle_client_fast_expiry;

                conn_expires_at = conn->idle_timestamp + expiry;

                now = READ_ONCE(jiffies);
                if (time_after(conn_expires_at, now))
                        goto not_yet_expired;
        }

        trace_rxrpc_client(conn, -1, rxrpc_client_discard);
        if (!test_and_clear_bit(RXRPC_CONN_EXPOSED, &conn->flags))
                BUG();
        conn->cache_state = RXRPC_CONN_CLIENT_INACTIVE;
        list_del_init(&conn->cache_link);

        spin_unlock(&rxrpc_client_conn_cache_lock);

        /* When we cleared the EXPOSED flag, we took on responsibility for the
         * reference that that had on the usage count.  We deal with that here.
         * If someone re-sets the flag and re-gets the ref, that's fine.
         */
        rxrpc_put_connection(conn);
        did_discard = true;
        nr_conns--;
        goto next;

not_yet_expired:
        /* The connection at the front of the queue hasn't yet expired, so
         * schedule the work item for that point if we discarded something.
         *
         * We don't worry if the work item is already scheduled - it can look
         * after rescheduling itself at a later time.  We could cancel it, but
         * then things get messier.
         */
        _debug("not yet");
        if (!rxrpc_kill_all_client_conns)
                queue_delayed_work(rxrpc_workqueue,
                                   &rxrpc_client_conn_reap,
                                   conn_expires_at - now);

out:
        spin_unlock(&rxrpc_client_conn_cache_lock);
        spin_unlock(&rxrpc_client_conn_discard_mutex);
        _leave("");
}

/*
 * Preemptively destroy all the client connection records rather than waiting
 * for them to time out
 */
void __exit rxrpc_destroy_all_client_connections(void)
{
        _enter("");

        spin_lock(&rxrpc_client_conn_cache_lock);
        rxrpc_kill_all_client_conns = true;
        spin_unlock(&rxrpc_client_conn_cache_lock);

        cancel_delayed_work(&rxrpc_client_conn_reap);

        if (!queue_delayed_work(rxrpc_workqueue, &rxrpc_client_conn_reap, 0))
                _debug("destroy: queue failed");

        _leave("");
}