net/core/dst.c - kernel/msm - Gitiles

 /*
  * net/core/dst.c	Protocol independent destination cache.
  *
  * Authors:		Alexey Kuznetsov, <kuznet@ms2.inr.ac.ru>
  *
  */

 #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 > 1)
 				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];

 void *dst_alloc(struct dst_ops *ops, struct net_device *dev,
 		int initial_ref, int initial_obsolete, int 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;
 	RCU_INIT_POINTER(dst->_neighbour, NULL);
 	dst->hh = NULL;
 #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->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 = 2;
 }

 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;
 		cancel_delayed_work(&dst_gc_work);
 		schedule_delayed_work(&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;
 	struct neighbour *neigh;
 	struct hh_cache *hh;

 	smp_rmb();

 again:
 	neigh = rcu_dereference_protected(dst->_neighbour, 1);
 	hh = dst->hh;
 	child = dst->child;

 	dst->hh = NULL;
 	if (hh)
 		hh_cache_put(hh);

 	if (neigh) {
 		RCU_INIT_POINTER(dst->_neighbour, NULL);
 		neigh_release(neigh);
 	}

 	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);

 /**
  * skb_dst_set_noref - sets skb dst, without a reference
  * @skb: buffer
  * @dst: dst entry
  *
  * Sets skb dst, assuming a reference was not taken on dst
  * skb_dst_drop() should not dst_release() this dst
  */
 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 {
 		struct neighbour *neigh;

 		dst->dev = dev_net(dst->dev)->loopback_dev;
 		dev_hold(dst->dev);
 		dev_put(dev);
 		rcu_read_lock();
 		neigh = dst_get_neighbour(dst);
 		if (neigh && neigh->dev == dev) {
 			neigh->dev = dst->dev;
 			dev_hold(dst->dev);
 			dev_put(dev);
 		}
 		rcu_read_unlock();
 	}
 }

 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:
 	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);
 }
	/*
	* net/core/dst.c Protocol independent destination cache.
	*
	* Authors: Alexey Kuznetsov, <kuznet@ms2.inr.ac.ru>
	*
	*/

	#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 > 1)
	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];

	void dst_alloc(struct dst_ops ops, struct net_device *dev,
	int initial_ref, int initial_obsolete, int 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;
	RCU_INIT_POINTER(dst->_neighbour, NULL);
	dst->hh = NULL;
	#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->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 = 2;
	}

	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;
	cancel_delayed_work(&dst_gc_work);
	schedule_delayed_work(&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;
	struct neighbour *neigh;
	struct hh_cache *hh;

	smp_rmb();

	again:
	neigh = rcu_dereference_protected(dst->_neighbour, 1);
	hh = dst->hh;
	child = dst->child;

	dst->hh = NULL;
	if (hh)
	hh_cache_put(hh);

	if (neigh) {
	RCU_INIT_POINTER(dst->_neighbour, NULL);
	neigh_release(neigh);
	}

	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);

	/**
	* skb_dst_set_noref - sets skb dst, without a reference
	* @skb: buffer
	* @dst: dst entry
	*
	* Sets skb dst, assuming a reference was not taken on dst
	* skb_dst_drop() should not dst_release() this dst
	*/
	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 {
	struct neighbour *neigh;

	dst->dev = dev_net(dst->dev)->loopback_dev;
	dev_hold(dst->dev);
	dev_put(dev);
	rcu_read_lock();
	neigh = dst_get_neighbour(dst);
	if (neigh && neigh->dev == dev) {
	neigh->dev = dst->dev;
	dev_hold(dst->dev);
	dev_put(dev);
	}
	rcu_read_unlock();
	}
	}

	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:
	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);
	}