inetpeer: remove AVL implementation in favor of RB tree

As discussed in Faro during Netfilter Workshop 2017, RB trees can be
used with RCU, using a seqlock.

Note that net/rxrpc/conn_service.c is already using this.

This patch converts inetpeer from AVL tree to RB tree, since it allows
to remove private AVL implementation in favor of shared RB code.

$ size net/ipv4/inetpeer.before net/ipv4/inetpeer.after
   text    data     bss     dec     hex filename
   3195      40     128    3363     d23 net/ipv4/inetpeer.before
   1562      24       0    1586     632 net/ipv4/inetpeer.after

The same technique can be used to speed up
net/netfilter/nft_set_rbtree.c (removing rwlock contention in fast path)
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/inetpeer.h

@@ -33,18 +33,12 @@ struct inetpeer_addr {
 };
 
 struct inet_peer {
-	/* group together avl_left,avl_right,v4daddr to speedup lookups */
-	struct inet_peer __rcu	*avl_left, *avl_right;
+	struct rb_node		rb_node;
 	struct inetpeer_addr	daddr;
-	__u32			avl_height;
 
 	u32			metrics[RTAX_MAX];
 	u32			rate_tokens;	/* rate limiting for ICMP */
 	unsigned long		rate_last;
-	union {
-		struct list_head	gc_list;
-		struct rcu_head     gc_rcu;
-	};
 	/*
 	 * Once inet_peer is queued for deletion (refcnt == 0), following field
 	 * is not available: rid
@@ -55,7 +49,6 @@ struct inet_peer {
 			atomic_t			rid;		/* Frag reception counter */
 		};
 		struct rcu_head         rcu;
-		struct inet_peer	*gc_next;
 	};
 
 	/* following fields might be frequently dirtied */
@@ -64,7 +57,7 @@ struct inet_peer {
 };
 
 struct inet_peer_base {
-	struct inet_peer __rcu	*root;
+	struct rb_root		rb_root;
 	seqlock_t		lock;
 	int			total;
 };

net/ipv4/inetpeer.c

@@ -33,7 +33,7 @@
  *  also be removed if the pool is overloaded i.e. if the total amount of
  *  entries is greater-or-equal than the threshold.
  *
- *  Node pool is organised as an AVL tree.
+ *  Node pool is organised as an RB tree.
  *  Such an implementation has been chosen not just for fun.  It's a way to
  *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
  *  amount of long living nodes in a single hash slot would significantly delay
@@ -45,7 +45,7 @@
  *      AND reference count being 0.
  *  3.  Global variable peer_total is modified under the pool lock.
  *  4.  struct inet_peer fields modification:
- *		avl_left, avl_right, avl_parent, avl_height: pool lock
+ *		rb_node: pool lock
  *		refcnt: atomically against modifications on other CPU;
  *		   usually under some other lock to prevent node disappearing
  *		daddr: unchangeable
@@ -53,30 +53,15 @@
 
 static struct kmem_cache *peer_cachep __read_mostly;
 
-static LIST_HEAD(gc_list);
-static const int gc_delay = 60 * HZ;
-static struct delayed_work gc_work;
-static DEFINE_SPINLOCK(gc_lock);
-
-#define node_height(x) x->avl_height
-
-#define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
-#define peer_avl_empty_rcu ((struct inet_peer __rcu __force *)&peer_fake_node)
-static const struct inet_peer peer_fake_node = {
-	.avl_left	= peer_avl_empty_rcu,
-	.avl_right	= peer_avl_empty_rcu,
-	.avl_height	= 0
-};
-
 void inet_peer_base_init(struct inet_peer_base *bp)
 {
-	bp->root = peer_avl_empty_rcu;
+	bp->rb_root = RB_ROOT;
 	seqlock_init(&bp->lock);
 	bp->total = 0;
 }
 EXPORT_SYMBOL_GPL(inet_peer_base_init);
 
-#define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */
+#define PEER_MAX_GC 32
 
 /* Exported for sysctl_net_ipv4.  */
 int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries more
@@ -84,53 +69,6 @@ int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries m
 int inet_peer_minttl __read_mostly = 120 * HZ;	/* TTL under high load: 120 sec */
 int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;	/* usual time to live: 10 min */
 
-static void inetpeer_gc_worker(struct work_struct *work)
-{
-	struct inet_peer *p, *n, *c;
-	struct list_head list;
-
-	spin_lock_bh(&gc_lock);
-	list_replace_init(&gc_list, &list);
-	spin_unlock_bh(&gc_lock);
-
-	if (list_empty(&list))
-		return;
-
-	list_for_each_entry_safe(p, n, &list, gc_list) {
-
-		if (need_resched())
-			cond_resched();
-
-		c = rcu_dereference_protected(p->avl_left, 1);
-		if (c != peer_avl_empty) {
-			list_add_tail(&c->gc_list, &list);
-			p->avl_left = peer_avl_empty_rcu;
-		}
-
-		c = rcu_dereference_protected(p->avl_right, 1);
-		if (c != peer_avl_empty) {
-			list_add_tail(&c->gc_list, &list);
-			p->avl_right = peer_avl_empty_rcu;
-		}
-
-		n = list_entry(p->gc_list.next, struct inet_peer, gc_list);
-
-		if (refcount_read(&p->refcnt) == 1) {
-			list_del(&p->gc_list);
-			kmem_cache_free(peer_cachep, p);
-		}
-	}
-
-	if (list_empty(&list))
-		return;
-
-	spin_lock_bh(&gc_lock);
-	list_splice(&list, &gc_list);
-	spin_unlock_bh(&gc_lock);
-
-	schedule_delayed_work(&gc_work, gc_delay);
-}
-
 /* Called from ip_output.c:ip_init  */
 void __init inet_initpeers(void)
 {
@@ -153,225 +91,62 @@ void __init inet_initpeers(void)
 			sizeof(struct inet_peer),
 			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC,
 			NULL);
-
-	INIT_DEFERRABLE_WORK(&gc_work, inetpeer_gc_worker);
 }
 
-#define rcu_deref_locked(X, BASE)				\
-	rcu_dereference_protected(X, lockdep_is_held(&(BASE)->lock.lock))
-
-/*
- * Called with local BH disabled and the pool lock held.
- */
-#define lookup(_daddr, _stack, _base)				\
-({								\
-	struct inet_peer *u;					\
-	struct inet_peer __rcu **v;				\
-								\
-	stackptr = _stack;					\
-	*stackptr++ = &_base->root;				\
-	for (u = rcu_deref_locked(_base->root, _base);		\
-	     u != peer_avl_empty;) {				\
-		int cmp = inetpeer_addr_cmp(_daddr, &u->daddr);	\
-		if (cmp == 0)					\
-			break;					\
-		if (cmp == -1)					\
-			v = &u->avl_left;			\
-		else						\
-			v = &u->avl_right;			\
-		*stackptr++ = v;				\
-		u = rcu_deref_locked(*v, _base);		\
-	}							\
-	u;							\
-})
-
-/*
- * Called with rcu_read_lock()
- * Because we hold no lock against a writer, its quite possible we fall
- * in an endless loop.
- * But every pointer we follow is guaranteed to be valid thanks to RCU.
- * We exit from this function if number of links exceeds PEER_MAXDEPTH
- */
-static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
-				    struct inet_peer_base *base)
+/* Called with rcu_read_lock() or base->lock held */
+static struct inet_peer *lookup(const struct inetpeer_addr *daddr,
+				struct inet_peer_base *base,
+				unsigned int seq,
+				struct inet_peer *gc_stack[],
+				unsigned int *gc_cnt,
+				struct rb_node **parent_p,
+				struct rb_node ***pp_p)
 {
-	struct inet_peer *u = rcu_dereference(base->root);
-	int count = 0;
+	struct rb_node **pp, *parent;
+	struct inet_peer *p;
+
+	pp = &base->rb_root.rb_node;
+	parent = NULL;
+	while (*pp) {
+		int cmp;
 
-	while (u != peer_avl_empty) {
-		int cmp = inetpeer_addr_cmp(daddr, &u->daddr);
+		parent = rcu_dereference_raw(*pp);
+		p = rb_entry(parent, struct inet_peer, rb_node);
+		cmp = inetpeer_addr_cmp(daddr, &p->daddr);
 		if (cmp == 0) {
-			/* Before taking a reference, check if this entry was
-			 * deleted (refcnt=0)
-			 */
-			if (!refcount_inc_not_zero(&u->refcnt)) {
-				u = NULL;
-			}
-			return u;
+			if (!refcount_inc_not_zero(&p->refcnt))
+				break;
+			return p;
+		}
+		if (gc_stack) {
+			if (*gc_cnt < PEER_MAX_GC)
+				gc_stack[(*gc_cnt)++] = p;
+		} else if (unlikely(read_seqretry(&base->lock, seq))) {
+			break;
 		}
 		if (cmp == -1)
-			u = rcu_dereference(u->avl_left);
+			pp = &(*pp)->rb_left;
 		else
-			u = rcu_dereference(u->avl_right);
-		if (unlikely(++count == PEER_MAXDEPTH))
-			break;
+			pp = &(*pp)->rb_right;
 	}
+	*parent_p = parent;
+	*pp_p = pp;
 	return NULL;
 }
 
-/* Called with local BH disabled and the pool lock held. */
-#define lookup_rightempty(start, base)				\
-({								\
-	struct inet_peer *u;					\
-	struct inet_peer __rcu **v;				\
-	*stackptr++ = &start->avl_left;				\
-	v = &start->avl_left;					\
-	for (u = rcu_deref_locked(*v, base);			\
-	     u->avl_right != peer_avl_empty_rcu;) {		\
-		v = &u->avl_right;				\
-		*stackptr++ = v;				\
-		u = rcu_deref_locked(*v, base);			\
-	}							\
-	u;							\
-})
-
-/* Called with local BH disabled and the pool lock held.
- * Variable names are the proof of operation correctness.
- * Look into mm/map_avl.c for more detail description of the ideas.
- */
-static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
-			       struct inet_peer __rcu ***stackend,
-			       struct inet_peer_base *base)
-{
-	struct inet_peer __rcu **nodep;
-	struct inet_peer *node, *l, *r;
-	int lh, rh;
-
-	while (stackend > stack) {
-		nodep = *--stackend;
-		node = rcu_deref_locked(*nodep, base);
-		l = rcu_deref_locked(node->avl_left, base);
-		r = rcu_deref_locked(node->avl_right, base);
-		lh = node_height(l);
-		rh = node_height(r);
-		if (lh > rh + 1) { /* l: RH+2 */
-			struct inet_peer *ll, *lr, *lrl, *lrr;
-			int lrh;
-			ll = rcu_deref_locked(l->avl_left, base);
-			lr = rcu_deref_locked(l->avl_right, base);
-			lrh = node_height(lr);
-			if (lrh <= node_height(ll)) {	/* ll: RH+1 */
-				RCU_INIT_POINTER(node->avl_left, lr);	/* lr: RH or RH+1 */
-				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
-				node->avl_height = lrh + 1; /* RH+1 or RH+2 */
-				RCU_INIT_POINTER(l->avl_left, ll);       /* ll: RH+1 */
-				RCU_INIT_POINTER(l->avl_right, node);	/* node: RH+1 or RH+2 */
-				l->avl_height = node->avl_height + 1;
-				RCU_INIT_POINTER(*nodep, l);
-			} else { /* ll: RH, lr: RH+1 */
-				lrl = rcu_deref_locked(lr->avl_left, base);/* lrl: RH or RH-1 */
-				lrr = rcu_deref_locked(lr->avl_right, base);/* lrr: RH or RH-1 */
-				RCU_INIT_POINTER(node->avl_left, lrr);	/* lrr: RH or RH-1 */
-				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
-				node->avl_height = rh + 1; /* node: RH+1 */
-				RCU_INIT_POINTER(l->avl_left, ll);	/* ll: RH */
-				RCU_INIT_POINTER(l->avl_right, lrl);	/* lrl: RH or RH-1 */
-				l->avl_height = rh + 1;	/* l: RH+1 */
-				RCU_INIT_POINTER(lr->avl_left, l);	/* l: RH+1 */
-				RCU_INIT_POINTER(lr->avl_right, node);	/* node: RH+1 */
-				lr->avl_height = rh + 2;
-				RCU_INIT_POINTER(*nodep, lr);
-			}
-		} else if (rh > lh + 1) { /* r: LH+2 */
-			struct inet_peer *rr, *rl, *rlr, *rll;
-			int rlh;
-			rr = rcu_deref_locked(r->avl_right, base);
-			rl = rcu_deref_locked(r->avl_left, base);
-			rlh = node_height(rl);
-			if (rlh <= node_height(rr)) {	/* rr: LH+1 */
-				RCU_INIT_POINTER(node->avl_right, rl);	/* rl: LH or LH+1 */
-				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
-				node->avl_height = rlh + 1; /* LH+1 or LH+2 */
-				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH+1 */
-				RCU_INIT_POINTER(r->avl_left, node);	/* node: LH+1 or LH+2 */
-				r->avl_height = node->avl_height + 1;
-				RCU_INIT_POINTER(*nodep, r);
-			} else { /* rr: RH, rl: RH+1 */
-				rlr = rcu_deref_locked(rl->avl_right, base);/* rlr: LH or LH-1 */
-				rll = rcu_deref_locked(rl->avl_left, base);/* rll: LH or LH-1 */
-				RCU_INIT_POINTER(node->avl_right, rll);	/* rll: LH or LH-1 */
-				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
-				node->avl_height = lh + 1; /* node: LH+1 */
-				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH */
-				RCU_INIT_POINTER(r->avl_left, rlr);	/* rlr: LH or LH-1 */
-				r->avl_height = lh + 1;	/* r: LH+1 */
-				RCU_INIT_POINTER(rl->avl_right, r);	/* r: LH+1 */
-				RCU_INIT_POINTER(rl->avl_left, node);	/* node: LH+1 */
-				rl->avl_height = lh + 2;
-				RCU_INIT_POINTER(*nodep, rl);
-			}
-		} else {
-			node->avl_height = (lh > rh ? lh : rh) + 1;
-		}
-	}
-}
-
-/* Called with local BH disabled and the pool lock held. */
-#define link_to_pool(n, base)					\
-do {								\
-	n->avl_height = 1;					\
-	n->avl_left = peer_avl_empty_rcu;			\
-	n->avl_right = peer_avl_empty_rcu;			\
-	/* lockless readers can catch us now */			\
-	rcu_assign_pointer(**--stackptr, n);			\
-	peer_avl_rebalance(stack, stackptr, base);		\
-} while (0)
-
 static void inetpeer_free_rcu(struct rcu_head *head)
 {
 	kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
 }
 
-static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base,
-			     struct inet_peer __rcu **stack[PEER_MAXDEPTH])
-{
-	struct inet_peer __rcu ***stackptr, ***delp;
-
-	if (lookup(&p->daddr, stack, base) != p)
-		BUG();
-	delp = stackptr - 1; /* *delp[0] == p */
-	if (p->avl_left == peer_avl_empty_rcu) {
-		*delp[0] = p->avl_right;
-		--stackptr;
-	} else {
-		/* look for a node to insert instead of p */
-		struct inet_peer *t;
-		t = lookup_rightempty(p, base);
-		BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
-		**--stackptr = t->avl_left;
-		/* t is removed, t->daddr > x->daddr for any
-		 * x in p->avl_left subtree.
-		 * Put t in the old place of p. */
-		RCU_INIT_POINTER(*delp[0], t);
-		t->avl_left = p->avl_left;
-		t->avl_right = p->avl_right;
-		t->avl_height = p->avl_height;
-		BUG_ON(delp[1] != &p->avl_left);
-		delp[1] = &t->avl_left; /* was &p->avl_left */
-	}
-	peer_avl_rebalance(stack, stackptr, base);
-	base->total--;
-	call_rcu(&p->rcu, inetpeer_free_rcu);
-}
-
 /* perform garbage collect on all items stacked during a lookup */
-static int inet_peer_gc(struct inet_peer_base *base,
-			struct inet_peer __rcu **stack[PEER_MAXDEPTH],
-			struct inet_peer __rcu ***stackptr)
+static void inet_peer_gc(struct inet_peer_base *base,
+			 struct inet_peer *gc_stack[],
+			 unsigned int gc_cnt)
 {
-	struct inet_peer *p, *gchead = NULL;
+	struct inet_peer *p;
 	__u32 delta, ttl;
-	int cnt = 0;
+	int i;
 
 	if (base->total >= inet_peer_threshold)
 		ttl = 0; /* be aggressive */
@@ -379,43 +154,38 @@ static int inet_peer_gc(struct inet_peer_base *base,
 		ttl = inet_peer_maxttl
 				- (inet_peer_maxttl - inet_peer_minttl) / HZ *
 					base->total / inet_peer_threshold * HZ;
-	stackptr--; /* last stack slot is peer_avl_empty */
-	while (stackptr > stack) {
-		stackptr--;
-		p = rcu_deref_locked(**stackptr, base);
-		if (refcount_read(&p->refcnt) == 1) {
-			smp_rmb();
-			delta = (__u32)jiffies - p->dtime;
-			if (delta >= ttl && refcount_dec_if_one(&p->refcnt)) {
-				p->gc_next = gchead;
-				gchead = p;
-			}
-		}
+	for (i = 0; i < gc_cnt; i++) {
+		p = gc_stack[i];
+		delta = (__u32)jiffies - p->dtime;
+		if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
+			gc_stack[i] = NULL;
 	}
-	while ((p = gchead) != NULL) {
-		gchead = p->gc_next;
-		cnt++;
-		unlink_from_pool(p, base, stack);
+	for (i = 0; i < gc_cnt; i++) {
+		p = gc_stack[i];
+		if (p) {
+			rb_erase(&p->rb_node, &base->rb_root);
+			base->total--;
+			call_rcu(&p->rcu, inetpeer_free_rcu);
+		}
 	}
-	return cnt;
 }
 
 struct inet_peer *inet_getpeer(struct inet_peer_base *base,
 			       const struct inetpeer_addr *daddr,
 			       int create)
 {
-	struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
-	struct inet_peer *p;
-	unsigned int sequence;
-	int invalidated, gccnt = 0;
+	struct inet_peer *p, *gc_stack[PEER_MAX_GC];
+	struct rb_node **pp, *parent;
+	unsigned int gc_cnt, seq;
+	int invalidated;
 
 	/* Attempt a lockless lookup first.
 	 * Because of a concurrent writer, we might not find an existing entry.
 	 */
 	rcu_read_lock();
-	sequence = read_seqbegin(&base->lock);
-	p = lookup_rcu(daddr, base);
-	invalidated = read_seqretry(&base->lock, sequence);
+	seq = read_seqbegin(&base->lock);
+	p = lookup(daddr, base, seq, NULL, &gc_cnt, &parent, &pp);
+	invalidated = read_seqretry(&base->lock, seq);
 	rcu_read_unlock();
 
 	if (p)
@@ -428,36 +198,31 @@ struct inet_peer *inet_getpeer(struct inet_peer_base *base,
 	/* retry an exact lookup, taking the lock before.
 	 * At least, nodes should be hot in our cache.
 	 */
+	parent = NULL;
 	write_seqlock_bh(&base->lock);
-relookup:
-	p = lookup(daddr, stack, base);
-	if (p != peer_avl_empty) {
-		refcount_inc(&p->refcnt);
-		write_sequnlock_bh(&base->lock);
-		return p;
-	}
-	if (!gccnt) {
-		gccnt = inet_peer_gc(base, stack, stackptr);
-		if (gccnt && create)
-			goto relookup;
-	}
-	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
-	if (p) {
-		p->daddr = *daddr;
-		refcount_set(&p->refcnt, 2);
-		atomic_set(&p->rid, 0);
-		p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
-		p->rate_tokens = 0;
-		/* 60*HZ is arbitrary, but chosen enough high so that the first
-		 * calculation of tokens is at its maximum.
-		 */
-		p->rate_last = jiffies - 60*HZ;
-		INIT_LIST_HEAD(&p->gc_list);
 
-		/* Link the node. */
-		link_to_pool(p, base);
-		base->total++;
+	gc_cnt = 0;
+	p = lookup(daddr, base, seq, gc_stack, &gc_cnt, &parent, &pp);
+	if (!p && create) {
+		p = kmem_cache_alloc(peer_cachep, GFP_ATOMIC);
+		if (p) {
+			p->daddr = *daddr;
+			refcount_set(&p->refcnt, 2);
+			atomic_set(&p->rid, 0);
+			p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
+			p->rate_tokens = 0;
+			/* 60*HZ is arbitrary, but chosen enough high so that the first
+			 * calculation of tokens is at its maximum.
+			 */
+			p->rate_last = jiffies - 60*HZ;
+
+			rb_link_node(&p->rb_node, parent, pp);
+			rb_insert_color(&p->rb_node, &base->rb_root);
+			base->total++;
+		}
 	}
+	if (gc_cnt)
+		inet_peer_gc(base, gc_stack, gc_cnt);
 	write_sequnlock_bh(&base->lock);
 
 	return p;
@@ -467,8 +232,9 @@ EXPORT_SYMBOL_GPL(inet_getpeer);
 void inet_putpeer(struct inet_peer *p)
 {
 	p->dtime = (__u32)jiffies;
-	smp_mb__before_atomic();
-	refcount_dec(&p->refcnt);
+
+	if (refcount_dec_and_test(&p->refcnt))
+		call_rcu(&p->rcu, inetpeer_free_rcu);
 }
 EXPORT_SYMBOL_GPL(inet_putpeer);
 
@@ -513,30 +279,16 @@ bool inet_peer_xrlim_allow(struct inet_peer *peer, int timeout)
 }
 EXPORT_SYMBOL(inet_peer_xrlim_allow);
 
-static void inetpeer_inval_rcu(struct rcu_head *head)
-{
-	struct inet_peer *p = container_of(head, struct inet_peer, gc_rcu);
-
-	spin_lock_bh(&gc_lock);
-	list_add_tail(&p->gc_list, &gc_list);
-	spin_unlock_bh(&gc_lock);
-
-	schedule_delayed_work(&gc_work, gc_delay);
-}
-
 void inetpeer_invalidate_tree(struct inet_peer_base *base)
 {
-	struct inet_peer *root;
-
-	write_seqlock_bh(&base->lock);
+	struct inet_peer *p, *n;
 
-	root = rcu_deref_locked(base->root, base);
-	if (root != peer_avl_empty) {
-		base->root = peer_avl_empty_rcu;
-		base->total = 0;
-		call_rcu(&root->gc_rcu, inetpeer_inval_rcu);
+	rbtree_postorder_for_each_entry_safe(p, n, &base->rb_root, rb_node) {
+		inet_putpeer(p);
+		cond_resched();
 	}
 
-	write_sequnlock_bh(&base->lock);
+	base->rb_root = RB_ROOT;
+	base->total = 0;
 }
 EXPORT_SYMBOL(inetpeer_invalidate_tree);