bond: Make the bond module thread safe.

[sliver-openvswitch.git] / lib / bond.c

/*
 * Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013 Nicira, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <config.h>

#include "bond.h"

#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <math.h>

#include "coverage.h"
#include "dynamic-string.h"
#include "flow.h"
#include "hmap.h"
#include "lacp.h"
#include "list.h"
#include "netdev.h"
#include "odp-util.h"
#include "ofpbuf.h"
#include "packets.h"
#include "poll-loop.h"
#include "shash.h"
#include "tag.h"
#include "timeval.h"
#include "unixctl.h"
#include "vlog.h"

VLOG_DEFINE_THIS_MODULE(bond);

/* Bit-mask for hashing a flow down to a bucket.
 * There are (BOND_MASK + 1) buckets. */
#define BOND_MASK 0xff

/* A hash bucket for mapping a flow to a slave.
 * "struct bond" has an array of (BOND_MASK + 1) of these. */
struct bond_entry {
    struct bond_slave *slave;   /* Assigned slave, NULL if unassigned. */
    uint64_t tx_bytes;          /* Count of bytes recently transmitted. */
    tag_type tag;               /* Tag for entry<->facet association. */
    struct list list_node;      /* In bond_slave's 'entries' list. */
};

/* A bond slave, that is, one of the links comprising a bond. */
struct bond_slave {
    struct hmap_node hmap_node; /* In struct bond's slaves hmap. */
    struct bond *bond;          /* The bond that contains this slave. */
    void *aux;                  /* Client-provided handle for this slave. */

    struct netdev *netdev;      /* Network device, owned by the client. */
    unsigned int change_seq;    /* Tracks changes in 'netdev'. */
    char *name;                 /* Name (a copy of netdev_get_name(netdev)). */

    /* Link status. */
    long long delay_expires;    /* Time after which 'enabled' may change. */
    bool enabled;               /* May be chosen for flows? */
    bool may_enable;            /* Client considers this slave bondable. */
    tag_type tag;               /* Tag associated with this slave. */

    /* Rebalancing info.  Used only by bond_rebalance(). */
    struct list bal_node;       /* In bond_rebalance()'s 'bals' list. */
    struct list entries;        /* 'struct bond_entry's assigned here. */
    uint64_t tx_bytes;          /* Sum across 'tx_bytes' of entries. */
};

/* A bond, that is, a set of network devices grouped to improve performance or
 * robustness.  */
struct bond {
    struct hmap_node hmap_node; /* In 'all_bonds' hmap. */
    char *name;                 /* Name provided by client. */

    /* Slaves. */
    struct hmap slaves;

    /* Bonding info. */
    enum bond_mode balance;     /* Balancing mode, one of BM_*. */
    struct bond_slave *active_slave;
    tag_type no_slaves_tag;     /* Tag for flows when all slaves disabled. */
    int updelay, downdelay;     /* Delay before slave goes up/down, in ms. */
    enum lacp_status lacp_status; /* Status of LACP negotiations. */
    bool bond_revalidate;       /* True if flows need revalidation. */
    uint32_t basis;             /* Basis for flow hash function. */

    /* SLB specific bonding info. */
    struct bond_entry *hash;     /* An array of (BOND_MASK + 1) elements. */
    int rebalance_interval;      /* Interval between rebalances, in ms. */
    long long int next_rebalance; /* Next rebalancing time. */
    bool send_learning_packets;

    /* Legacy compatibility. */
    long long int next_fake_iface_update; /* LLONG_MAX if disabled. */

    /* Tag set saved for next bond_run().  This tag set is a kluge for cases
     * where we can't otherwise provide revalidation feedback to the client.
     * That's only unixctl commands now; I hope no other cases will arise. */
    struct tag_set unixctl_tags;

    atomic_int ref_cnt;
};

static struct ovs_rwlock rwlock = OVS_RWLOCK_INITIALIZER;
static struct hmap all_bonds__ = HMAP_INITIALIZER(&all_bonds__);
static struct hmap *const all_bonds OVS_GUARDED_BY(rwlock) = &all_bonds__;

static void bond_entry_reset(struct bond *) OVS_REQ_WRLOCK(rwlock);
static struct bond_slave *bond_slave_lookup(struct bond *, const void *slave_)
    OVS_REQ_RDLOCK(rwlock);
static void bond_enable_slave(struct bond_slave *, bool enable,
                              struct tag_set *) OVS_REQ_WRLOCK(rwlock);
static void bond_link_status_update(struct bond_slave *, struct tag_set *)
    OVS_REQ_WRLOCK(rwlock);
static void bond_choose_active_slave(struct bond *, struct tag_set *)
    OVS_REQ_WRLOCK(rwlock);;
static unsigned int bond_hash_src(const uint8_t mac[ETH_ADDR_LEN],
                                  uint16_t vlan, uint32_t basis);
static unsigned int bond_hash_tcp(const struct flow *, uint16_t vlan,
                                  uint32_t basis);
static struct bond_entry *lookup_bond_entry(const struct bond *,
                                            const struct flow *,
                                            uint16_t vlan)
    OVS_REQ_RDLOCK(rwlock);
static tag_type bond_get_active_slave_tag(const struct bond *)
    OVS_REQ_RDLOCK(rwlock);
static struct bond_slave *choose_output_slave(const struct bond *,
                                              const struct flow *,
                                              struct flow_wildcards *,
                                              uint16_t vlan, tag_type *tags)
    OVS_REQ_RDLOCK(rwlock);
static void bond_update_fake_slave_stats(struct bond *)
    OVS_REQ_RDLOCK(rwlock);

/* Attempts to parse 's' as the name of a bond balancing mode.  If successful,
 * stores the mode in '*balance' and returns true.  Otherwise returns false
 * without modifying '*balance'. */
bool
bond_mode_from_string(enum bond_mode *balance, const char *s)
{
    if (!strcmp(s, bond_mode_to_string(BM_TCP))) {
        *balance = BM_TCP;
    } else if (!strcmp(s, bond_mode_to_string(BM_SLB))) {
        *balance = BM_SLB;
    } else if (!strcmp(s, bond_mode_to_string(BM_AB))) {
        *balance = BM_AB;
    } else {
        return false;
    }
    return true;
}

/* Returns a string representing 'balance'. */
const char *
bond_mode_to_string(enum bond_mode balance) {
    switch (balance) {
    case BM_TCP:
        return "balance-tcp";
    case BM_SLB:
        return "balance-slb";
    case BM_AB:
        return "active-backup";
    }
    NOT_REACHED();
}

\f
/* Creates and returns a new bond whose configuration is initially taken from
 * 's'.
 *
 * The caller should register each slave on the new bond by calling
 * bond_slave_register().  */
struct bond *
bond_create(const struct bond_settings *s)
{
    struct bond *bond;

    bond = xzalloc(sizeof *bond);
    hmap_init(&bond->slaves);
    bond->no_slaves_tag = tag_create_random();
    bond->next_fake_iface_update = LLONG_MAX;
    atomic_init(&bond->ref_cnt, 1);

    bond_reconfigure(bond, s);

    tag_set_init(&bond->unixctl_tags);

    return bond;
}

struct bond *
bond_ref(const struct bond *bond_)
{
    struct bond *bond = CONST_CAST(struct bond *, bond_);

    if (bond) {
        int orig;
        atomic_add(&bond->ref_cnt, 1, &orig);
        ovs_assert(orig > 0);
    }
    return bond;
}

/* Frees 'bond'. */
void
bond_unref(struct bond *bond)
{
    struct bond_slave *slave, *next_slave;
    int orig;

    if (!bond) {
        return;
    }

    atomic_sub(&bond->ref_cnt, 1, &orig);
    ovs_assert(orig > 0);
    if (orig != 1) {
        return;
    }

    ovs_rwlock_wrlock(&rwlock);
    hmap_remove(all_bonds, &bond->hmap_node);
    ovs_rwlock_unlock(&rwlock);

    HMAP_FOR_EACH_SAFE (slave, next_slave, hmap_node, &bond->slaves) {
        hmap_remove(&bond->slaves, &slave->hmap_node);
        /* Client owns 'slave->netdev'. */
        free(slave->name);
        free(slave);
    }
    hmap_destroy(&bond->slaves);

    free(bond->hash);
    free(bond->name);
    free(bond);
}

/* Updates 'bond''s overall configuration to 's'.
 *
 * The caller should register each slave on 'bond' by calling
 * bond_slave_register().  This is optional if none of the slaves'
 * configuration has changed.  In any case it can't hurt.
 *
 * Returns true if the configuration has changed in such a way that requires
 * flow revalidation.
 * */
bool
bond_reconfigure(struct bond *bond, const struct bond_settings *s)
{
    bool revalidate = false;

    ovs_rwlock_wrlock(&rwlock);
    if (!bond->name || strcmp(bond->name, s->name)) {
        if (bond->name) {
            hmap_remove(all_bonds, &bond->hmap_node);
            free(bond->name);
        }
        bond->name = xstrdup(s->name);
        hmap_insert(all_bonds, &bond->hmap_node, hash_string(bond->name, 0));
    }

    bond->updelay = s->up_delay;
    bond->downdelay = s->down_delay;

    if (bond->rebalance_interval != s->rebalance_interval) {
        bond->rebalance_interval = s->rebalance_interval;
        revalidate = true;
    }

    if (bond->balance != s->balance) {
        bond->balance = s->balance;
        revalidate = true;
    }

    if (bond->basis != s->basis) {
        bond->basis = s->basis;
        revalidate = true;
    }

    if (s->fake_iface) {
        if (bond->next_fake_iface_update == LLONG_MAX) {
            bond->next_fake_iface_update = time_msec();
        }
    } else {
        bond->next_fake_iface_update = LLONG_MAX;
    }

    if (bond->bond_revalidate) {
        revalidate = true;
        bond->bond_revalidate = false;
    }

    if (bond->balance == BM_AB || !bond->hash || revalidate) {
        bond_entry_reset(bond);
    }

    ovs_rwlock_unlock(&rwlock);
    return revalidate;
}

static void
bond_slave_set_netdev__(struct bond_slave *slave, struct netdev *netdev)
    OVS_REQ_WRLOCK(rwlock)
{
    if (slave->netdev != netdev) {
        slave->netdev = netdev;
        slave->change_seq = 0;
    }
}

/* Registers 'slave_' as a slave of 'bond'.  The 'slave_' pointer is an
 * arbitrary client-provided pointer that uniquely identifies a slave within a
 * bond.  If 'slave_' already exists within 'bond' then this function
 * reconfigures the existing slave.
 *
 * 'netdev' must be the network device that 'slave_' represents.  It is owned
 * by the client, so the client must not close it before either unregistering
 * 'slave_' or destroying 'bond'.
 */
void
bond_slave_register(struct bond *bond, void *slave_, struct netdev *netdev)
{
    struct bond_slave *slave;

    ovs_rwlock_wrlock(&rwlock);
    slave = bond_slave_lookup(bond, slave_);
    if (!slave) {
        slave = xzalloc(sizeof *slave);

        hmap_insert(&bond->slaves, &slave->hmap_node, hash_pointer(slave_, 0));
        slave->bond = bond;
        slave->aux = slave_;
        slave->delay_expires = LLONG_MAX;
        slave->name = xstrdup(netdev_get_name(netdev));
        bond->bond_revalidate = true;

        slave->enabled = false;
        bond_enable_slave(slave, netdev_get_carrier(netdev), NULL);
    }

    bond_slave_set_netdev__(slave, netdev);

    free(slave->name);
    slave->name = xstrdup(netdev_get_name(netdev));
    ovs_rwlock_unlock(&rwlock);
}

/* Updates the network device to be used with 'slave_' to 'netdev'.
 *
 * This is useful if the caller closes and re-opens the network device
 * registered with bond_slave_register() but doesn't need to change anything
 * else. */
void
bond_slave_set_netdev(struct bond *bond, void *slave_, struct netdev *netdev)
{
    struct bond_slave *slave;

    ovs_rwlock_wrlock(&rwlock);
    slave = bond_slave_lookup(bond, slave_);
    if (slave) {
        bond_slave_set_netdev__(slave, netdev);
    }
    ovs_rwlock_unlock(&rwlock);
}

/* Unregisters 'slave_' from 'bond'.  If 'bond' does not contain such a slave
 * then this function has no effect.
 *
 * Unregistering a slave invalidates all flows. */
void
bond_slave_unregister(struct bond *bond, const void *slave_)
{
    struct bond_slave *slave;
    bool del_active;

    ovs_rwlock_wrlock(&rwlock);
    slave = bond_slave_lookup(bond, slave_);
    if (!slave) {
        goto out;
    }

    bond_enable_slave(slave, false, NULL);

    del_active = bond->active_slave == slave;
    if (bond->hash) {
        struct bond_entry *e;
        for (e = bond->hash; e <= &bond->hash[BOND_MASK]; e++) {
            if (e->slave == slave) {
                e->slave = NULL;
            }
        }
    }

    free(slave->name);

    hmap_remove(&bond->slaves, &slave->hmap_node);
    /* Client owns 'slave->netdev'. */
    free(slave);

    if (del_active) {
        struct tag_set tags;

        tag_set_init(&tags);
        bond_choose_active_slave(bond, &tags);
        bond->send_learning_packets = true;
    }
out:
    ovs_rwlock_unlock(&rwlock);
}

/* Should be called on each slave in 'bond' before bond_run() to indicate
 * whether or not 'slave_' may be enabled. This function is intended to allow
 * other protocols to have some impact on bonding decisions.  For example LACP
 * or high level link monitoring protocols may decide that a given slave should
 * not be able to send traffic. */
void
bond_slave_set_may_enable(struct bond *bond, void *slave_, bool may_enable)
{
    ovs_rwlock_wrlock(&rwlock);
    bond_slave_lookup(bond, slave_)->may_enable = may_enable;
    ovs_rwlock_unlock(&rwlock);
}

/* Performs periodic maintenance on 'bond'.  The caller must provide 'tags' to
 * allow tagged flows to be invalidated.
 *
 * The caller should check bond_should_send_learning_packets() afterward. */
void
bond_run(struct bond *bond, struct tag_set *tags, enum lacp_status lacp_status)
{
    struct bond_slave *slave;

    ovs_rwlock_wrlock(&rwlock);
    if (bond->lacp_status != lacp_status) {
        bond->lacp_status = lacp_status;
        bond->bond_revalidate = true;
    }

    /* Enable slaves based on link status and LACP feedback. */
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        bond_link_status_update(slave, tags);
        slave->change_seq = netdev_change_seq(slave->netdev);
    }
    if (!bond->active_slave || !bond->active_slave->enabled) {
        bond_choose_active_slave(bond, tags);
    }

    /* Update fake bond interface stats. */
    if (time_msec() >= bond->next_fake_iface_update) {
        bond_update_fake_slave_stats(bond);
        bond->next_fake_iface_update = time_msec() + 1000;
    }

    if (bond->bond_revalidate) {
        struct bond_slave *slave;

        bond->bond_revalidate = false;
        bond_entry_reset(bond);
        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
            tag_set_add(tags, slave->tag);
        }
        tag_set_add(tags, bond->no_slaves_tag);
    }

    /* Invalidate any tags required by  */
    tag_set_union(tags, &bond->unixctl_tags);
    tag_set_init(&bond->unixctl_tags);
    ovs_rwlock_unlock(&rwlock);
}

/* Causes poll_block() to wake up when 'bond' needs something to be done. */
void
bond_wait(struct bond *bond)
{
    struct bond_slave *slave;

    ovs_rwlock_rdlock(&rwlock);
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        if (slave->delay_expires != LLONG_MAX) {
            poll_timer_wait_until(slave->delay_expires);
        }

        if (slave->change_seq != netdev_change_seq(slave->netdev)) {
            poll_immediate_wake();
        }
    }

    if (bond->next_fake_iface_update != LLONG_MAX) {
        poll_timer_wait_until(bond->next_fake_iface_update);
    }

    /* Ensure that any saved tags get revalidated right away. */
    if (!tag_set_is_empty(&bond->unixctl_tags)) {
        poll_immediate_wake();
    }
    ovs_rwlock_unlock(&rwlock);

    /* We don't wait for bond->next_rebalance because rebalancing can only run
     * at a flow account checkpoint.  ofproto does checkpointing on its own
     * schedule and bond_rebalance() gets called afterward, so we'd just be
     * waking up for no purpose. */
}
\f
/* MAC learning table interaction. */

static bool
may_send_learning_packets(const struct bond *bond)
{
    return bond->lacp_status == LACP_DISABLED
        && (bond->balance == BM_SLB || bond->balance == BM_AB)
        && bond->active_slave;
}

/* Returns true if 'bond' needs the client to send out packets to assist with
 * MAC learning on 'bond'.  If this function returns true, then the client
 * should iterate through its MAC learning table for the bridge on which 'bond'
 * is located.  For each MAC that has been learned on a port other than 'bond',
 * it should call bond_compose_learning_packet().
 *
 * This function will only return true if 'bond' is in SLB or active-backup
 * mode and LACP is not negotiated.  Otherwise sending learning packets isn't
 * necessary.
 *
 * Calling this function resets the state that it checks. */
bool
bond_should_send_learning_packets(struct bond *bond)
{
    bool send;

    ovs_rwlock_wrlock(&rwlock);
    send = bond->send_learning_packets && may_send_learning_packets(bond);
    bond->send_learning_packets = false;
    ovs_rwlock_unlock(&rwlock);
    return send;
}

/* Sends a gratuitous learning packet on 'bond' from 'eth_src' on 'vlan'.
 *
 * See bond_should_send_learning_packets() for description of usage. The
 * caller should send the composed packet on the port associated with
 * port_aux and takes ownership of the returned ofpbuf. */
struct ofpbuf *
bond_compose_learning_packet(struct bond *bond,
                             const uint8_t eth_src[ETH_ADDR_LEN],
                             uint16_t vlan, void **port_aux)
{
    struct bond_slave *slave;
    struct ofpbuf *packet;
    tag_type tags = 0;
    struct flow flow;

    ovs_rwlock_rdlock(&rwlock);
    ovs_assert(may_send_learning_packets(bond));
    memset(&flow, 0, sizeof flow);
    memcpy(flow.dl_src, eth_src, ETH_ADDR_LEN);
    slave = choose_output_slave(bond, &flow, NULL, vlan, &tags);

    packet = ofpbuf_new(0);
    compose_rarp(packet, eth_src);
    if (vlan) {
        eth_push_vlan(packet, htons(vlan));
    }

    *port_aux = slave->aux;
    ovs_rwlock_unlock(&rwlock);
    return packet;
}
\f
/* Checks whether a packet that arrived on 'slave_' within 'bond', with an
 * Ethernet destination address of 'eth_dst', should be admitted.
 *
 * The return value is one of the following:
 *
 *    - BV_ACCEPT: Admit the packet.
 *
 *    - BV_DROP: Drop the packet.
 *
 *    - BV_DROP_IF_MOVED: Consult the MAC learning table for the packet's
 *      Ethernet source address and VLAN.  If there is none, or if the packet
 *      is on the learned port, then admit the packet.  If a different port has
 *      been learned, however, drop the packet (and do not use it for MAC
 *      learning).
 */
enum bond_verdict
bond_check_admissibility(struct bond *bond, const void *slave_,
                         const uint8_t eth_dst[ETH_ADDR_LEN], tag_type *tags)
{
    enum bond_verdict verdict = BV_DROP;
    struct bond_slave *slave;

    ovs_rwlock_rdlock(&rwlock);
    slave = bond_slave_lookup(bond, slave_);
    if (!slave) {
        goto out;
    }

    /* LACP bonds have very loose admissibility restrictions because we can
     * assume the remote switch is aware of the bond and will "do the right
     * thing".  However, as a precaution we drop packets on disabled slaves
     * because no correctly implemented partner switch should be sending
     * packets to them.
     *
     * If LACP is configured, but LACP negotiations have been unsuccessful, we
     * drop all incoming traffic. */
    switch (bond->lacp_status) {
    case LACP_NEGOTIATED:
        verdict = slave->enabled ? BV_ACCEPT : BV_DROP;
        goto out;
    case LACP_CONFIGURED:
        goto out;
    case LACP_DISABLED:
        break;
    }

    /* Drop all multicast packets on inactive slaves. */
    if (eth_addr_is_multicast(eth_dst)) {
        *tags |= bond_get_active_slave_tag(bond);
        if (bond->active_slave != slave) {
            goto out;
        }
    }

    switch (bond->balance) {
    case BM_AB:
        /* Drop all packets which arrive on backup slaves.  This is similar to
         * how Linux bonding handles active-backup bonds. */
        *tags |= bond_get_active_slave_tag(bond);
        if (bond->active_slave != slave) {
            static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(1, 5);

            VLOG_DBG_RL(&rl, "active-backup bond received packet on backup"
                        " slave (%s) destined for " ETH_ADDR_FMT,
                        slave->name, ETH_ADDR_ARGS(eth_dst));
            goto out;
        }
        verdict = BV_ACCEPT;
        goto out;

    case BM_TCP:
        /* TCP balanced bonds require successful LACP negotiated. Based on the
         * above check, LACP is off on this bond.  Therfore, we drop all
         * incoming traffic. */
        goto out;

    case BM_SLB:
        /* Drop all packets for which we have learned a different input port,
         * because we probably sent the packet on one slave and got it back on
         * the other.  Gratuitous ARP packets are an exception to this rule:
         * the host has moved to another switch.  The exception to the
         * exception is if we locked the learning table to avoid reflections on
         * bond slaves. */
        verdict = BV_DROP_IF_MOVED;
        goto out;
    }

    NOT_REACHED();
out:
    ovs_rwlock_unlock(&rwlock);
    return verdict;

}

/* Returns the slave (registered on 'bond' by bond_slave_register()) to which
 * a packet with the given 'flow' and 'vlan' should be forwarded.  Returns
 * NULL if the packet should be dropped because no slaves are enabled.
 *
 * 'vlan' is not necessarily the same as 'flow->vlan_tci'.  First, 'vlan'
 * should be a VID only (i.e. excluding the PCP bits).  Second,
 * 'flow->vlan_tci' is the VLAN TCI that appeared on the packet (so it will be
 * nonzero only for trunk ports), whereas 'vlan' is the logical VLAN that the
 * packet belongs to (so for an access port it will be the access port's VLAN).
 *
 * Adds a tag to '*tags' that associates the flow with the returned slave.
 *
 * If 'wc' is non-NULL, bitwise-OR's 'wc' with the set of bits that were
 * significant in the selection.  At some point earlier, 'wc' should
 * have been initialized (e.g., by flow_wildcards_init_catchall()).
 */
void *
bond_choose_output_slave(struct bond *bond, const struct flow *flow,
                         struct flow_wildcards *wc, uint16_t vlan,
                         tag_type *tags)
{
    struct bond_slave *slave;
    void *result = NULL;

    ovs_rwlock_rdlock(&rwlock);
    slave = choose_output_slave(bond, flow, wc, vlan, tags);
    if (slave) {
        *tags |= slave->tag;
        result = slave->aux;
    } else {
        *tags |= bond->no_slaves_tag;
    }
    ovs_rwlock_unlock(&rwlock);
    return result;
}
\f
/* Rebalancing. */

static bool
bond_is_balanced(const struct bond *bond) OVS_REQ_RDLOCK(rwlock)
{
    return bond->rebalance_interval
        && (bond->balance == BM_SLB || bond->balance == BM_TCP);
}

/* Notifies 'bond' that 'n_bytes' bytes were sent in 'flow' within 'vlan'. */
void
bond_account(struct bond *bond, const struct flow *flow, uint16_t vlan,
             uint64_t n_bytes)
{
    ovs_rwlock_wrlock(&rwlock);
    if (bond_is_balanced(bond)) {
        lookup_bond_entry(bond, flow, vlan)->tx_bytes += n_bytes;
    }
    ovs_rwlock_unlock(&rwlock);
}

static struct bond_slave *
bond_slave_from_bal_node(struct list *bal) OVS_REQ_RDLOCK(rwlock)
{
    return CONTAINER_OF(bal, struct bond_slave, bal_node);
}

static void
log_bals(struct bond *bond, const struct list *bals)
{
    if (VLOG_IS_DBG_ENABLED()) {
        struct ds ds = DS_EMPTY_INITIALIZER;
        const struct bond_slave *slave;

        LIST_FOR_EACH (slave, bal_node, bals) {
            if (ds.length) {
                ds_put_char(&ds, ',');
            }
            ds_put_format(&ds, " %s %"PRIu64"kB",
                          slave->name, slave->tx_bytes / 1024);

            if (!slave->enabled) {
                ds_put_cstr(&ds, " (disabled)");
            }
            if (!list_is_empty(&slave->entries)) {
                struct bond_entry *e;

                ds_put_cstr(&ds, " (");
                LIST_FOR_EACH (e, list_node, &slave->entries) {
                    if (&e->list_node != list_front(&slave->entries)) {
                        ds_put_cstr(&ds, " + ");
                    }
                    ds_put_format(&ds, "h%td: %"PRIu64"kB",
                                  e - bond->hash, e->tx_bytes / 1024);
                }
                ds_put_cstr(&ds, ")");
            }
        }
        VLOG_DBG("bond %s:%s", bond->name, ds_cstr(&ds));
        ds_destroy(&ds);
    }
}

/* Shifts 'hash' from its current slave to 'to'. */
static void
bond_shift_load(struct bond_entry *hash, struct bond_slave *to,
                struct tag_set *set)
{
    struct bond_slave *from = hash->slave;
    struct bond *bond = from->bond;
    uint64_t delta = hash->tx_bytes;

    VLOG_INFO("bond %s: shift %"PRIu64"kB of load (with hash %td) "
              "from %s to %s (now carrying %"PRIu64"kB and "
              "%"PRIu64"kB load, respectively)",
              bond->name, delta / 1024, hash - bond->hash,
              from->name, to->name,
              (from->tx_bytes - delta) / 1024,
              (to->tx_bytes + delta) / 1024);

    /* Shift load away from 'from' to 'to'. */
    from->tx_bytes -= delta;
    to->tx_bytes += delta;

    /* Arrange for flows to be revalidated. */
    tag_set_add(set, hash->tag);
    hash->slave = to;
    hash->tag = tag_create_random();
}

/* Picks and returns a bond_entry to migrate from 'from' (the most heavily
 * loaded bond slave) to a bond slave that has 'to_tx_bytes' bytes of load,
 * given that doing so must decrease the ratio of the load on the two slaves by
 * at least 0.1.  Returns NULL if there is no appropriate entry.
 *
 * The list of entries isn't sorted.  I don't know of a reason to prefer to
 * shift away small hashes or large hashes. */
static struct bond_entry *
choose_entry_to_migrate(const struct bond_slave *from, uint64_t to_tx_bytes)
{
    struct bond_entry *e;

    if (list_is_short(&from->entries)) {
        /* 'from' carries no more than one MAC hash, so shifting load away from
         * it would be pointless. */
        return NULL;
    }

    LIST_FOR_EACH (e, list_node, &from->entries) {
        double old_ratio, new_ratio;
        uint64_t delta;

        if (to_tx_bytes == 0) {
            /* Nothing on the new slave, move it. */
            return e;
        }

        delta = e->tx_bytes;
        old_ratio = (double)from->tx_bytes / to_tx_bytes;
        new_ratio = (double)(from->tx_bytes - delta) / (to_tx_bytes + delta);
        if (old_ratio - new_ratio > 0.1
            && fabs(new_ratio - 1.0) < fabs(old_ratio - 1.0)) {
            /* We're aiming for an ideal ratio of 1, meaning both the 'from'
               and 'to' slave have the same load.  Therefore, we only move an
               entry if it decreases the load on 'from', and brings us closer
               to equal traffic load. */
            return e;
        }
    }

    return NULL;
}

/* Inserts 'slave' into 'bals' so that descending order of 'tx_bytes' is
 * maintained. */
static void
insert_bal(struct list *bals, struct bond_slave *slave)
{
    struct bond_slave *pos;

    LIST_FOR_EACH (pos, bal_node, bals) {
        if (slave->tx_bytes > pos->tx_bytes) {
            break;
        }
    }
    list_insert(&pos->bal_node, &slave->bal_node);
}

/* Removes 'slave' from its current list and then inserts it into 'bals' so
 * that descending order of 'tx_bytes' is maintained. */
static void
reinsert_bal(struct list *bals, struct bond_slave *slave)
{
    list_remove(&slave->bal_node);
    insert_bal(bals, slave);
}

/* If 'bond' needs rebalancing, does so.
 *
 * The caller should have called bond_account() for each active flow, to ensure
 * that flow data is consistently accounted at this point. */
void
bond_rebalance(struct bond *bond, struct tag_set *tags)
{
    struct bond_slave *slave;
    struct bond_entry *e;
    struct list bals;

    ovs_rwlock_wrlock(&rwlock);
    if (!bond_is_balanced(bond) || time_msec() < bond->next_rebalance) {
        ovs_rwlock_unlock(&rwlock);
        return;
    }
    bond->next_rebalance = time_msec() + bond->rebalance_interval;

    /* Add each bond_entry to its slave's 'entries' list.
     * Compute each slave's tx_bytes as the sum of its entries' tx_bytes. */
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        slave->tx_bytes = 0;
        list_init(&slave->entries);
    }
    for (e = &bond->hash[0]; e <= &bond->hash[BOND_MASK]; e++) {
        if (e->slave && e->tx_bytes) {
            e->slave->tx_bytes += e->tx_bytes;
            list_push_back(&e->slave->entries, &e->list_node);
        }
    }

    /* Add enabled slaves to 'bals' in descending order of tx_bytes.
     *
     * XXX This is O(n**2) in the number of slaves but it could be O(n lg n)
     * with a proper list sort algorithm. */
    list_init(&bals);
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        if (slave->enabled) {
            insert_bal(&bals, slave);
        }
    }
    log_bals(bond, &bals);

    /* Shift load from the most-loaded slaves to the least-loaded slaves. */
    while (!list_is_short(&bals)) {
        struct bond_slave *from = bond_slave_from_bal_node(list_front(&bals));
        struct bond_slave *to = bond_slave_from_bal_node(list_back(&bals));
        uint64_t overload;

        overload = from->tx_bytes - to->tx_bytes;
        if (overload < to->tx_bytes >> 5 || overload < 100000) {
            /* The extra load on 'from' (and all less-loaded slaves), compared
             * to that of 'to' (the least-loaded slave), is less than ~3%, or
             * it is less than ~1Mbps.  No point in rebalancing. */
            break;
        }

        /* 'from' is carrying significantly more load than 'to'.  Pick a hash
         * to move from 'from' to 'to'. */
        e = choose_entry_to_migrate(from, to->tx_bytes);
        if (e) {
            bond_shift_load(e, to, tags);

            /* Delete element from from->entries.
             *
             * We don't add the element to to->hashes.  That would only allow
             * 'e' to be migrated to another slave in this rebalancing run, and
             * there is no point in doing that. */
            list_remove(&e->list_node);

            /* Re-sort 'bals'. */
            reinsert_bal(&bals, from);
            reinsert_bal(&bals, to);
        } else {
            /* Can't usefully migrate anything away from 'from'.
             * Don't reconsider it. */
            list_remove(&from->bal_node);
        }
    }

    /* Implement exponentially weighted moving average.  A weight of 1/2 causes
     * historical data to decay to <1% in 7 rebalancing runs.  1,000,000 bytes
     * take 20 rebalancing runs to decay to 0 and get deleted entirely. */
    for (e = &bond->hash[0]; e <= &bond->hash[BOND_MASK]; e++) {
        e->tx_bytes /= 2;
        if (!e->tx_bytes) {
            e->slave = NULL;
        }
    }
    ovs_rwlock_unlock(&rwlock);
}
\f
/* Bonding unixctl user interface functions. */

static struct bond *
bond_find(const char *name) OVS_REQ_RDLOCK(rwlock)
{
    struct bond *bond;

    HMAP_FOR_EACH_WITH_HASH (bond, hmap_node, hash_string(name, 0),
                             all_bonds) {
        if (!strcmp(bond->name, name)) {
            return bond;
        }
    }
    return NULL;
}

static struct bond_slave *
bond_lookup_slave(struct bond *bond, const char *slave_name)
{
    struct bond_slave *slave;

    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        if (!strcmp(slave->name, slave_name)) {
            return slave;
        }
    }
    return NULL;
}

static void
bond_unixctl_list(struct unixctl_conn *conn,
                  int argc OVS_UNUSED, const char *argv[] OVS_UNUSED,
                  void *aux OVS_UNUSED)
{
    struct ds ds = DS_EMPTY_INITIALIZER;
    const struct bond *bond;

    ds_put_cstr(&ds, "bond\ttype\tslaves\n");

    ovs_rwlock_rdlock(&rwlock);
    HMAP_FOR_EACH (bond, hmap_node, all_bonds) {
        const struct bond_slave *slave;
        size_t i;

        ds_put_format(&ds, "%s\t%s\t",
                      bond->name, bond_mode_to_string(bond->balance));

        i = 0;
        HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
            if (i++ > 0) {
                ds_put_cstr(&ds, ", ");
            }
            ds_put_cstr(&ds, slave->name);
        }
        ds_put_char(&ds, '\n');
    }
    ovs_rwlock_unlock(&rwlock);
    unixctl_command_reply(conn, ds_cstr(&ds));
    ds_destroy(&ds);
}

static void
bond_print_details(struct ds *ds, const struct bond *bond)
    OVS_REQ_RDLOCK(rwlock)
{
    struct shash slave_shash = SHASH_INITIALIZER(&slave_shash);
    const struct shash_node **sorted_slaves = NULL;
    const struct bond_slave *slave;
    int i;

    ds_put_format(ds, "---- %s ----\n", bond->name);
    ds_put_format(ds, "bond_mode: %s\n",
                  bond_mode_to_string(bond->balance));

    ds_put_format(ds, "bond-hash-basis: %"PRIu32"\n", bond->basis);

    ds_put_format(ds, "updelay: %d ms\n", bond->updelay);
    ds_put_format(ds, "downdelay: %d ms\n", bond->downdelay);

    if (bond_is_balanced(bond)) {
        ds_put_format(ds, "next rebalance: %lld ms\n",
                      bond->next_rebalance - time_msec());
    }

    ds_put_cstr(ds, "lacp_status: ");
    switch (bond->lacp_status) {
    case LACP_NEGOTIATED:
        ds_put_cstr(ds, "negotiated\n");
        break;
    case LACP_CONFIGURED:
        ds_put_cstr(ds, "configured\n");
        break;
    case LACP_DISABLED:
        ds_put_cstr(ds, "off\n");
        break;
    default:
        ds_put_cstr(ds, "<unknown>\n");
        break;
    }

    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        shash_add(&slave_shash, slave->name, slave);
    }
    sorted_slaves = shash_sort(&slave_shash);

    for (i = 0; i < shash_count(&slave_shash); i++) {
        struct bond_entry *be;

        slave = sorted_slaves[i]->data;

        /* Basic info. */
        ds_put_format(ds, "\nslave %s: %s\n",
                      slave->name, slave->enabled ? "enabled" : "disabled");
        if (slave == bond->active_slave) {
            ds_put_cstr(ds, "\tactive slave\n");
        }
        if (slave->delay_expires != LLONG_MAX) {
            ds_put_format(ds, "\t%s expires in %lld ms\n",
                          slave->enabled ? "downdelay" : "updelay",
                          slave->delay_expires - time_msec());
        }

        ds_put_format(ds, "\tmay_enable: %s\n",
                      slave->may_enable ? "true" : "false");

        if (!bond_is_balanced(bond)) {
            continue;
        }

        /* Hashes. */
        for (be = bond->hash; be <= &bond->hash[BOND_MASK]; be++) {
            int hash = be - bond->hash;

            if (be->slave != slave) {
                continue;
            }

            ds_put_format(ds, "\thash %d: %"PRIu64" kB load\n",
                          hash, be->tx_bytes / 1024);

            /* XXX How can we list the MACs assigned to hashes of SLB bonds? */
        }
    }
    shash_destroy(&slave_shash);
    free(sorted_slaves);
    ds_put_cstr(ds, "\n");
}

static void
bond_unixctl_show(struct unixctl_conn *conn,
                  int argc, const char *argv[],
                  void *aux OVS_UNUSED)
{
    struct ds ds = DS_EMPTY_INITIALIZER;

    ovs_rwlock_rdlock(&rwlock);
    if (argc > 1) {
        const struct bond *bond = bond_find(argv[1]);

        if (!bond) {
            unixctl_command_reply_error(conn, "no such bond");
            goto out;
        }
        bond_print_details(&ds, bond);
    } else {
        const struct bond *bond;

        HMAP_FOR_EACH (bond, hmap_node, all_bonds) {
            bond_print_details(&ds, bond);
        }
    }

    unixctl_command_reply(conn, ds_cstr(&ds));
    ds_destroy(&ds);

out:
    ovs_rwlock_unlock(&rwlock);
}

static void
bond_unixctl_migrate(struct unixctl_conn *conn,
                     int argc OVS_UNUSED, const char *argv[],
                     void *aux OVS_UNUSED)
{
    const char *bond_s = argv[1];
    const char *hash_s = argv[2];
    const char *slave_s = argv[3];
    struct bond *bond;
    struct bond_slave *slave;
    struct bond_entry *entry;
    int hash;

    ovs_rwlock_wrlock(&rwlock);
    bond = bond_find(bond_s);
    if (!bond) {
        unixctl_command_reply_error(conn, "no such bond");
        goto out;
    }

    if (bond->balance != BM_SLB) {
        unixctl_command_reply_error(conn, "not an SLB bond");
        goto out;
    }

    if (strspn(hash_s, "0123456789") == strlen(hash_s)) {
        hash = atoi(hash_s) & BOND_MASK;
    } else {
        unixctl_command_reply_error(conn, "bad hash");
        goto out;
    }

    slave = bond_lookup_slave(bond, slave_s);
    if (!slave) {
        unixctl_command_reply_error(conn, "no such slave");
        goto out;
    }

    if (!slave->enabled) {
        unixctl_command_reply_error(conn, "cannot migrate to disabled slave");
        goto out;
    }

    entry = &bond->hash[hash];
    tag_set_add(&bond->unixctl_tags, entry->tag);
    entry->slave = slave;
    entry->tag = tag_create_random();
    unixctl_command_reply(conn, "migrated");

out:
    ovs_rwlock_unlock(&rwlock);
}

static void
bond_unixctl_set_active_slave(struct unixctl_conn *conn,
                              int argc OVS_UNUSED, const char *argv[],
                              void *aux OVS_UNUSED)
{
    const char *bond_s = argv[1];
    const char *slave_s = argv[2];
    struct bond *bond;
    struct bond_slave *slave;

    ovs_rwlock_wrlock(&rwlock);
    bond = bond_find(bond_s);
    if (!bond) {
        unixctl_command_reply_error(conn, "no such bond");
        goto out;
    }

    slave = bond_lookup_slave(bond, slave_s);
    if (!slave) {
        unixctl_command_reply_error(conn, "no such slave");
        goto out;
    }

    if (!slave->enabled) {
        unixctl_command_reply_error(conn, "cannot make disabled slave active");
        goto out;
    }

    if (bond->active_slave != slave) {
        tag_set_add(&bond->unixctl_tags, bond_get_active_slave_tag(bond));
        bond->active_slave = slave;
        bond->active_slave->tag = tag_create_random();
        VLOG_INFO("bond %s: active interface is now %s",
                  bond->name, slave->name);
        bond->send_learning_packets = true;
        unixctl_command_reply(conn, "done");
    } else {
        unixctl_command_reply(conn, "no change");
    }
out:
    ovs_rwlock_unlock(&rwlock);
}

static void
enable_slave(struct unixctl_conn *conn, const char *argv[], bool enable)
{
    const char *bond_s = argv[1];
    const char *slave_s = argv[2];
    struct bond *bond;
    struct bond_slave *slave;

    ovs_rwlock_wrlock(&rwlock);
    bond = bond_find(bond_s);
    if (!bond) {
        unixctl_command_reply_error(conn, "no such bond");
        goto out;
    }

    slave = bond_lookup_slave(bond, slave_s);
    if (!slave) {
        unixctl_command_reply_error(conn, "no such slave");
        goto out;
    }

    bond_enable_slave(slave, enable, &bond->unixctl_tags);
    unixctl_command_reply(conn, enable ? "enabled" : "disabled");

out:
    ovs_rwlock_unlock(&rwlock);
}

static void
bond_unixctl_enable_slave(struct unixctl_conn *conn,
                          int argc OVS_UNUSED, const char *argv[],
                          void *aux OVS_UNUSED)
{
    enable_slave(conn, argv, true);
}

static void
bond_unixctl_disable_slave(struct unixctl_conn *conn,
                           int argc OVS_UNUSED, const char *argv[],
                           void *aux OVS_UNUSED)
{
    enable_slave(conn, argv, false);
}

static void
bond_unixctl_hash(struct unixctl_conn *conn, int argc, const char *argv[],
                  void *aux OVS_UNUSED)
{
    const char *mac_s = argv[1];
    const char *vlan_s = argc > 2 ? argv[2] : NULL;
    const char *basis_s = argc > 3 ? argv[3] : NULL;
    uint8_t mac[ETH_ADDR_LEN];
    uint8_t hash;
    char *hash_cstr;
    unsigned int vlan;
    uint32_t basis;

    if (vlan_s) {
        if (sscanf(vlan_s, "%u", &vlan) != 1) {
            unixctl_command_reply_error(conn, "invalid vlan");
            return;
        }
    } else {
        vlan = 0;
    }

    if (basis_s) {
        if (sscanf(basis_s, "%"PRIu32, &basis) != 1) {
            unixctl_command_reply_error(conn, "invalid basis");
            return;
        }
    } else {
        basis = 0;
    }

    if (sscanf(mac_s, ETH_ADDR_SCAN_FMT, ETH_ADDR_SCAN_ARGS(mac))
        == ETH_ADDR_SCAN_COUNT) {
        hash = bond_hash_src(mac, vlan, basis) & BOND_MASK;

        hash_cstr = xasprintf("%u", hash);
        unixctl_command_reply(conn, hash_cstr);
        free(hash_cstr);
    } else {
        unixctl_command_reply_error(conn, "invalid mac");
    }
}

void
bond_init(void)
{
    unixctl_command_register("bond/list", "", 0, 0, bond_unixctl_list, NULL);
    unixctl_command_register("bond/show", "[port]", 0, 1, bond_unixctl_show,
                             NULL);
    unixctl_command_register("bond/migrate", "port hash slave", 3, 3,
                             bond_unixctl_migrate, NULL);
    unixctl_command_register("bond/set-active-slave", "port slave", 2, 2,
                             bond_unixctl_set_active_slave, NULL);
    unixctl_command_register("bond/enable-slave", "port slave", 2, 2,
                             bond_unixctl_enable_slave, NULL);
    unixctl_command_register("bond/disable-slave", "port slave", 2, 2,
                             bond_unixctl_disable_slave, NULL);
    unixctl_command_register("bond/hash", "mac [vlan] [basis]", 1, 3,
                             bond_unixctl_hash, NULL);
}
\f
static void
bond_entry_reset(struct bond *bond)
{
    if (bond->balance != BM_AB) {
        size_t hash_len = (BOND_MASK + 1) * sizeof *bond->hash;

        if (!bond->hash) {
            bond->hash = xmalloc(hash_len);
        }
        memset(bond->hash, 0, hash_len);

        bond->next_rebalance = time_msec() + bond->rebalance_interval;
    } else {
        free(bond->hash);
        bond->hash = NULL;
    }
}

static struct bond_slave *
bond_slave_lookup(struct bond *bond, const void *slave_)
{
    struct bond_slave *slave;

    HMAP_FOR_EACH_IN_BUCKET (slave, hmap_node, hash_pointer(slave_, 0),
                             &bond->slaves) {
        if (slave->aux == slave_) {
            return slave;
        }
    }

    return NULL;
}

static void
bond_enable_slave(struct bond_slave *slave, bool enable, struct tag_set *tags)
{
    slave->delay_expires = LLONG_MAX;
    if (enable != slave->enabled) {
        slave->enabled = enable;
        if (!slave->enabled) {
            VLOG_INFO("interface %s: disabled", slave->name);
            if (tags) {
                tag_set_add(tags, slave->tag);
            }
        } else {
            VLOG_INFO("interface %s: enabled", slave->name);
            slave->tag = tag_create_random();
        }
    }
}

static void
bond_link_status_update(struct bond_slave *slave, struct tag_set *tags)
{
    struct bond *bond = slave->bond;
    bool up;

    up = netdev_get_carrier(slave->netdev) && slave->may_enable;
    if ((up == slave->enabled) != (slave->delay_expires == LLONG_MAX)) {
        static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
        VLOG_INFO_RL(&rl, "interface %s: link state %s",
                     slave->name, up ? "up" : "down");
        if (up == slave->enabled) {
            slave->delay_expires = LLONG_MAX;
            VLOG_INFO_RL(&rl, "interface %s: will not be %s",
                         slave->name, up ? "disabled" : "enabled");
        } else {
            int delay = (bond->lacp_status != LACP_DISABLED ? 0
                         : up ? bond->updelay : bond->downdelay);
            slave->delay_expires = time_msec() + delay;
            if (delay) {
                VLOG_INFO_RL(&rl, "interface %s: will be %s if it stays %s "
                             "for %d ms",
                             slave->name,
                             up ? "enabled" : "disabled",
                             up ? "up" : "down",
                             delay);
            }
        }
    }

    if (time_msec() >= slave->delay_expires) {
        bond_enable_slave(slave, up, tags);
    }
}

static unsigned int
bond_hash_src(const uint8_t mac[ETH_ADDR_LEN], uint16_t vlan, uint32_t basis)
{
    return hash_3words(hash_bytes(mac, ETH_ADDR_LEN, 0), vlan, basis);
}

static unsigned int
bond_hash_tcp(const struct flow *flow, uint16_t vlan, uint32_t basis)
{
    struct flow hash_flow = *flow;
    hash_flow.vlan_tci = htons(vlan);

    /* The symmetric quality of this hash function is not required, but
     * flow_hash_symmetric_l4 already exists, and is sufficient for our
     * purposes, so we use it out of convenience. */
    return flow_hash_symmetric_l4(&hash_flow, basis);
}

static unsigned int
bond_hash(const struct bond *bond, const struct flow *flow, uint16_t vlan)
{
    ovs_assert(bond->balance == BM_TCP || bond->balance == BM_SLB);

    return (bond->balance == BM_TCP
            ? bond_hash_tcp(flow, vlan, bond->basis)
            : bond_hash_src(flow->dl_src, vlan, bond->basis));
}

static struct bond_entry *
lookup_bond_entry(const struct bond *bond, const struct flow *flow,
                  uint16_t vlan)
{
    return &bond->hash[bond_hash(bond, flow, vlan) & BOND_MASK];
}

static struct bond_slave *
choose_output_slave(const struct bond *bond, const struct flow *flow,
                    struct flow_wildcards *wc, uint16_t vlan, tag_type *tags)
{
    struct bond_entry *e;

    if (bond->lacp_status == LACP_CONFIGURED) {
        /* LACP has been configured on this bond but negotiations were
         * unsuccussful.  Drop all traffic. */
        return NULL;
    }

    switch (bond->balance) {
    case BM_AB:
        return bond->active_slave;

    case BM_TCP:
        if (bond->lacp_status != LACP_NEGOTIATED) {
            /* Must have LACP negotiations for TCP balanced bonds. */
            return NULL;
        }
        if (wc) {
            flow_mask_hash_fields(flow, wc, NX_HASH_FIELDS_SYMMETRIC_L4);
        }
        /* Fall Through. */
    case BM_SLB:
        if (wc) {
            flow_mask_hash_fields(flow, wc, NX_HASH_FIELDS_ETH_SRC);
        }
        e = lookup_bond_entry(bond, flow, vlan);
        if (!e->slave || !e->slave->enabled) {
            e->slave = CONTAINER_OF(hmap_random_node(&bond->slaves),
                                    struct bond_slave, hmap_node);
            if (!e->slave->enabled) {
                e->slave = bond->active_slave;
            }
            e->tag = tag_create_random();
        }
        *tags |= e->tag;
        return e->slave;

    default:
        NOT_REACHED();
    }
}

static struct bond_slave *
bond_choose_slave(const struct bond *bond)
{
    struct bond_slave *slave, *best;

    /* Find an enabled slave. */
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        if (slave->enabled) {
            return slave;
        }
    }

    /* All interfaces are disabled.  Find an interface that will be enabled
     * after its updelay expires.  */
    best = NULL;
    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        if (slave->delay_expires != LLONG_MAX
            && slave->may_enable
            && (!best || slave->delay_expires < best->delay_expires)) {
            best = slave;
        }
    }
    return best;
}

static void
bond_choose_active_slave(struct bond *bond, struct tag_set *tags)
{
    static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
    struct bond_slave *old_active_slave = bond->active_slave;

    bond->active_slave = bond_choose_slave(bond);
    if (bond->active_slave) {
        if (bond->active_slave->enabled) {
            VLOG_INFO_RL(&rl, "bond %s: active interface is now %s",
                         bond->name, bond->active_slave->name);
        } else {
            VLOG_INFO_RL(&rl, "bond %s: active interface is now %s, skipping "
                         "remaining %lld ms updelay (since no interface was "
                         "enabled)", bond->name, bond->active_slave->name,
                         bond->active_slave->delay_expires - time_msec());
            bond_enable_slave(bond->active_slave, true, tags);
        }

        if (!old_active_slave) {
            tag_set_add(tags, bond->no_slaves_tag);
        }

        bond->send_learning_packets = true;
    } else if (old_active_slave) {
        VLOG_INFO_RL(&rl, "bond %s: all interfaces disabled", bond->name);
    }
}

/* Returns the tag for 'bond''s active slave, or 'bond''s no_slaves_tag if
 * there is no active slave. */
static tag_type
bond_get_active_slave_tag(const struct bond *bond)
{
    return (bond->active_slave
            ? bond->active_slave->tag
            : bond->no_slaves_tag);
}

/* Attempts to make the sum of the bond slaves' statistics appear on the fake
 * bond interface. */
static void
bond_update_fake_slave_stats(struct bond *bond)
{
    struct netdev_stats bond_stats;
    struct bond_slave *slave;
    struct netdev *bond_dev;

    memset(&bond_stats, 0, sizeof bond_stats);

    HMAP_FOR_EACH (slave, hmap_node, &bond->slaves) {
        struct netdev_stats slave_stats;

        if (!netdev_get_stats(slave->netdev, &slave_stats)) {
            /* XXX: We swap the stats here because they are swapped back when
             * reported by the internal device.  The reason for this is
             * internal devices normally represent packets going into the
             * system but when used as fake bond device they represent packets
             * leaving the system.  We really should do this in the internal
             * device itself because changing it here reverses the counts from
             * the perspective of the switch.  However, the internal device
             * doesn't know what type of device it represents so we have to do
             * it here for now. */
            bond_stats.tx_packets += slave_stats.rx_packets;
            bond_stats.tx_bytes += slave_stats.rx_bytes;
            bond_stats.rx_packets += slave_stats.tx_packets;
            bond_stats.rx_bytes += slave_stats.tx_bytes;
        }
    }

    if (!netdev_open(bond->name, "system", &bond_dev)) {
        netdev_set_stats(bond_dev, &bond_stats);
        netdev_close(bond_dev);
    }
}