dst.c

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/*
 * net/core/dst.c   Protocol independent destination cache.
 *
 * Authors:     Alexey Kuznetsov, <[email protected]>
 *
 */

#include <linux/bitops.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/workqueue.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/netdevice.h>
#include <linux/skbuff.h>
#include <linux/string.h>
#include <linux/types.h>
#include <net/net_namespace.h>
#include <linux/sched.h>
#include <linux/prefetch.h>

#include <net/dst.h>

/*
 * Theory of operations:
 * 1) We use a list, protected by a spinlock, to add
 *    new entries from both BH and non-BH context.
 * 2) In order to keep spinlock held for a small delay,
 *    we use a second list where are stored long lived
 *    entries, that are handled by the garbage collect thread
 *    fired by a workqueue.
 * 3) This list is guarded by a mutex,
 *    so that the gc_task and dst_dev_event() can be synchronized.
 */

/*
 * We want to keep lock & list close together
 * to dirty as few cache lines as possible in __dst_free().
 * As this is not a very strong hint, we dont force an alignment on SMP.
 */
static struct {
    spinlock_t      lock;
    struct dst_entry    *list;
    unsigned long       timer_inc;
    unsigned long       timer_expires;
} dst_garbage = {
    .lock = __SPIN_LOCK_UNLOCKED(dst_garbage.lock),
    .timer_inc = DST_GC_MAX,
};
static void dst_gc_task(struct work_struct *work);
static void ___dst_free(struct dst_entry *dst);

static DECLARE_DELAYED_WORK(dst_gc_work, dst_gc_task);

static DEFINE_MUTEX(dst_gc_mutex);
/*
 * long lived entries are maintained in this list, guarded by dst_gc_mutex
 */
static struct dst_entry         *dst_busy_list;

static void dst_gc_task(struct work_struct *work)
{
    int    delayed = 0;
    int    work_performed = 0;
    unsigned long expires = ~0L;
    struct dst_entry *dst, *next, head;
    struct dst_entry *last = &head;

    mutex_lock(&dst_gc_mutex);
    next = dst_busy_list;

loop:
    while ((dst = next) != NULL) {
        next = dst->next;
        prefetch(&next->next);
        cond_resched();
        if (likely(atomic_read(&dst->__refcnt))) {
            last->next = dst;
            last = dst;
            delayed++;
            continue;
        }
        work_performed++;

        dst = dst_destroy(dst);
        if (dst) {
            /* NOHASH and still referenced. Unless it is already
             * on gc list, invalidate it and add to gc list.
             *
             * Note: this is temporary. Actually, NOHASH dst's
             * must be obsoleted when parent is obsoleted.
             * But we do not have state "obsoleted, but
             * referenced by parent", so it is right.
             */
            if (dst->obsolete > 0)
                continue;

            ___dst_free(dst);
            dst->next = next;
            next = dst;
        }
    }

    spin_lock_bh(&dst_garbage.lock);
    next = dst_garbage.list;
    if (next) {
        dst_garbage.list = NULL;
        spin_unlock_bh(&dst_garbage.lock);
        goto loop;
    }
    last->next = NULL;
    dst_busy_list = head.next;
    if (!dst_busy_list)
        dst_garbage.timer_inc = DST_GC_MAX;
    else {
        /*
         * if we freed less than 1/10 of delayed entries,
         * we can sleep longer.
         */
        if (work_performed <= delayed/10) {
            dst_garbage.timer_expires += dst_garbage.timer_inc;
            if (dst_garbage.timer_expires > DST_GC_MAX)
                dst_garbage.timer_expires = DST_GC_MAX;
            dst_garbage.timer_inc += DST_GC_INC;
        } else {
            dst_garbage.timer_inc = DST_GC_INC;
            dst_garbage.timer_expires = DST_GC_MIN;
        }
        expires = dst_garbage.timer_expires;
        /*
         * if the next desired timer is more than 4 seconds in the
         * future then round the timer to whole seconds
         */
        if (expires > 4*HZ)
            expires = round_jiffies_relative(expires);
        schedule_delayed_work(&dst_gc_work, expires);
    }

    spin_unlock_bh(&dst_garbage.lock);
    mutex_unlock(&dst_gc_mutex);
}

int dst_discard(struct sk_buff *skb)
{
    kfree_skb(skb);
    return 0;
}
EXPORT_SYMBOL(dst_discard);

const u32 dst_default_metrics[RTAX_MAX + 1] = {
    /* This initializer is needed to force linker to place this variable
     * into const section. Otherwise it might end into bss section.
     * We really want to avoid false sharing on this variable, and catch
     * any writes on it.
     */
    [RTAX_MAX] = 0xdeadbeef,
};


void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
        int initial_ref, int initial_obsolete, unsigned short flags)
{
    struct dst_entry *dst;

    if (ops->gc && dst_entries_get_fast(ops) > ops->gc_thresh) {
        if (ops->gc(ops))
            return NULL;
    }
    dst = kmem_cache_alloc(ops->kmem_cachep, GFP_ATOMIC);
    if (!dst)
        return NULL;
    dst->child = NULL;
    dst->dev = dev;
    if (dev)
        dev_hold(dev);
    dst->ops = ops;
    dst_init_metrics(dst, dst_default_metrics, true);
    dst->expires = 0UL;
    dst->path = dst;
#ifdef CONFIG_XFRM
    dst->xfrm = NULL;
#endif
    dst->input = dst_discard;
    dst->output = dst_discard;
    dst->error = 0;
    dst->obsolete = initial_obsolete;
    dst->header_len = 0;
    dst->trailer_len = 0;
#ifdef CONFIG_IP_ROUTE_CLASSID
    dst->tclassid = 0;
#endif
    atomic_set(&dst->__refcnt, initial_ref);
    dst->__use = 0;
    dst->lastuse = jiffies;
    dst->flags = flags;
    dst->pending_confirm = 0;
    dst->next = NULL;
    if (!(flags & DST_NOCOUNT))
        dst_entries_add(ops, 1);
    return dst;
}
EXPORT_SYMBOL(dst_alloc);

static void ___dst_free(struct dst_entry *dst)
{
    /* The first case (dev==NULL) is required, when
       protocol module is unloaded.
     */
    if (dst->dev == NULL || !(dst->dev->flags&IFF_UP))
        dst->input = dst->output = dst_discard;
    dst->obsolete = DST_OBSOLETE_DEAD;
}

void __dst_free(struct dst_entry *dst)
{
    spin_lock_bh(&dst_garbage.lock);
    ___dst_free(dst);
    dst->next = dst_garbage.list;
    dst_garbage.list = dst;
    if (dst_garbage.timer_inc > DST_GC_INC) {
        dst_garbage.timer_inc = DST_GC_INC;
        dst_garbage.timer_expires = DST_GC_MIN;
        mod_delayed_work(system_wq, &dst_gc_work,
                 dst_garbage.timer_expires);
    }
    spin_unlock_bh(&dst_garbage.lock);
}
EXPORT_SYMBOL(__dst_free);

struct dst_entry *dst_destroy(struct dst_entry * dst)
{
    struct dst_entry *child;

    smp_rmb();

again:
    child = dst->child;

    if (!(dst->flags & DST_NOCOUNT))
        dst_entries_add(dst->ops, -1);

    if (dst->ops->destroy)
        dst->ops->destroy(dst);
    if (dst->dev)
        dev_put(dst->dev);
    kmem_cache_free(dst->ops->kmem_cachep, dst);

    dst = child;
    if (dst) {
        int nohash = dst->flags & DST_NOHASH;

        if (atomic_dec_and_test(&dst->__refcnt)) {
            /* We were real parent of this dst, so kill child. */
            if (nohash)
                goto again;
        } else {
            /* Child is still referenced, return it for freeing. */
            if (nohash)
                return dst;
            /* Child is still in his hash table */
        }
    }
    return NULL;
}
EXPORT_SYMBOL(dst_destroy);

void dst_release(struct dst_entry *dst)
{
    if (dst) {
        int newrefcnt;

        newrefcnt = atomic_dec_return(&dst->__refcnt);
        WARN_ON(newrefcnt < 0);
        if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {
            dst = dst_destroy(dst);
            if (dst)
                __dst_free(dst);
        }
    }
}
EXPORT_SYMBOL(dst_release);

u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old)
{
    u32 *p = kmalloc(sizeof(u32) * RTAX_MAX, GFP_ATOMIC);

    if (p) {
        u32 *old_p = __DST_METRICS_PTR(old);
        unsigned long prev, new;

        memcpy(p, old_p, sizeof(u32) * RTAX_MAX);

        new = (unsigned long) p;
        prev = cmpxchg(&dst->_metrics, old, new);

        if (prev != old) {
            kfree(p);
            p = __DST_METRICS_PTR(prev);
            if (prev & DST_METRICS_READ_ONLY)
                p = NULL;
        }
    }
    return p;
}
EXPORT_SYMBOL(dst_cow_metrics_generic);

/* Caller asserts that dst_metrics_read_only(dst) is false.  */
void __dst_destroy_metrics_generic(struct dst_entry *dst, unsigned long old)
{
    unsigned long prev, new;

    new = ((unsigned long) dst_default_metrics) | DST_METRICS_READ_ONLY;
    prev = cmpxchg(&dst->_metrics, old, new);
    if (prev == old)
        kfree(__DST_METRICS_PTR(old));
}
EXPORT_SYMBOL(__dst_destroy_metrics_generic);

void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
{
    WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
    /* If dst not in cache, we must take a reference, because
     * dst_release() will destroy dst as soon as its refcount becomes zero
     */
    if (unlikely(dst->flags & DST_NOCACHE)) {
        dst_hold(dst);
        skb_dst_set(skb, dst);
    } else {
        skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
    }
}
EXPORT_SYMBOL(skb_dst_set_noref);

/* Dirty hack. We did it in 2.2 (in __dst_free),
 * we have _very_ good reasons not to repeat
 * this mistake in 2.3, but we have no choice
 * now. _It_ _is_ _explicit_ _deliberate_
 * _race_ _condition_.
 *
 * Commented and originally written by Alexey.
 */
static void dst_ifdown(struct dst_entry *dst, struct net_device *dev,
               int unregister)
{
    if (dst->ops->ifdown)
        dst->ops->ifdown(dst, dev, unregister);

    if (dev != dst->dev)
        return;

    if (!unregister) {
        dst->input = dst->output = dst_discard;
    } else {
        dst->dev = dev_net(dst->dev)->loopback_dev;
        dev_hold(dst->dev);
        dev_put(dev);
    }
}

static int dst_dev_event(struct notifier_block *this, unsigned long event,
             void *ptr)
{
    struct net_device *dev = ptr;
    struct dst_entry *dst, *last = NULL;

    switch (event) {
    case NETDEV_UNREGISTER_FINAL:
    case NETDEV_DOWN:
        mutex_lock(&dst_gc_mutex);
        for (dst = dst_busy_list; dst; dst = dst->next) {
            last = dst;
            dst_ifdown(dst, dev, event != NETDEV_DOWN);
        }

        spin_lock_bh(&dst_garbage.lock);
        dst = dst_garbage.list;
        dst_garbage.list = NULL;
        spin_unlock_bh(&dst_garbage.lock);

        if (last)
            last->next = dst;
        else
            dst_busy_list = dst;
        for (; dst; dst = dst->next)
            dst_ifdown(dst, dev, event != NETDEV_DOWN);
        mutex_unlock(&dst_gc_mutex);
        break;
    }
    return NOTIFY_DONE;
}

static struct notifier_block dst_dev_notifier = {
    .notifier_call  = dst_dev_event,
    .priority = -10, /* must be called after other network notifiers */
};

void __init dst_init(void)
{
    register_netdevice_notifier(&dst_dev_notifier);
}