Message-id: <20080926031832.GA27289@minyard.local>
Date:	Thu, 25 Sep 2008 22:18:33 -0500
From:	Corey Minyard <minyard@....org>
To:	Linux Kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org
Cc:	jarkao2@...il.com, paulmck@...ux.vnet.ibm.com,
	shemminger@...tta.com
Subject: [PATCH 1/1] Use RCU for the UDP hash lock

From: Corey Minyard <cminyard@...sta.com>

Convert access to the udp_hash table to use RCU.

Signed-off-by: Corey Minyard <cminyard@...sta.com>
---
 include/linux/rculist.h |   19 +++++++++++++++++
 include/net/sock.h      |   51 +++++++++++++++++++++++++++++++++++++++++++++++
 include/net/udp.h       |    9 ++++---
 net/ipv4/udp.c          |   47 ++++++++++++++++++++++++------------------
 net/ipv6/udp.c          |   17 ++++++++-------
 5 files changed, 111 insertions(+), 32 deletions(-)

This patch is the second try; I believe I fixed all issues that people
raised.  Thanks to everyone who commented on this.

I beat on this for a few hours with my test program, too.

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index eb4443c..4d3cc58 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -397,5 +397,24 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
 		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); \
 		pos = rcu_dereference(pos->next))
 
+
+/**
+ * hlist_for_each_entry_from_rcu - iterate over rcu list starting from pos
+ * @tpos:      the type * to use as a loop cursor.
+ * @pos:       the &struct hlist_node to use as a loop cursor.
+ * @head:      the head for your list.
+ * @member:    the name of the hlist_node within the struct.
+ *
+ * This list-traversal primitive may safely run concurrently with
+ * the _rcu list-mutation primitives such as hlist_add_head_rcu()
+ * as long as the traversal is guarded by rcu_read_lock().
+ */
+#define hlist_for_each_entry_from_rcu(tpos, pos, member)                \
+	for (;                                                          \
+	     rcu_dereference(pos) && ({ prefetch(pos->next); 1; }) &&    \
+	       ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); \
+	     pos = pos->next)
+
+
 #endif	/* __KERNEL__ */
 #endif
diff --git a/include/net/sock.h b/include/net/sock.h
index 06c5259..65110a6 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -42,6 +42,7 @@
 
 #include <linux/kernel.h>
 #include <linux/list.h>
+#include <linux/rculist.h>
 #include <linux/timer.h>
 #include <linux/cache.h>
 #include <linux/module.h>
@@ -294,12 +295,24 @@ static inline struct sock *sk_head(const struct hlist_head *head)
 	return hlist_empty(head) ? NULL : __sk_head(head);
 }
 
+static inline struct sock *sk_head_rcu(const struct hlist_head *head)
+{
+	struct hlist_node *first = rcu_dereference(head->first);
+	return first ? hlist_entry(first, struct sock, sk_node) : NULL;
+}
+
 static inline struct sock *sk_next(const struct sock *sk)
 {
 	return sk->sk_node.next ?
 		hlist_entry(sk->sk_node.next, struct sock, sk_node) : NULL;
 }
 
+static inline struct sock *sk_next_rcu(const struct sock *sk)
+{
+	struct hlist_node *next = rcu_dereference(sk->sk_node.next);
+	return next ? hlist_entry(next, struct sock, sk_node) : NULL;
+}
+
 static inline int sk_unhashed(const struct sock *sk)
 {
 	return hlist_unhashed(&sk->sk_node);
@@ -361,6 +374,27 @@ static __inline__ int sk_del_node_init(struct sock *sk)
 	return rc;
 }
 
+static inline int __sk_del_node_rcu(struct sock *sk)
+{
+	if (sk_hashed(sk)) {
+		hlist_del_rcu(&sk->sk_node);
+		return 1;
+	}
+	return 0;
+}
+
+static inline int sk_del_node_rcu(struct sock *sk)
+{
+	int rc = __sk_del_node_rcu(sk);
+
+	if (rc) {
+		/* paranoid for a while -acme */
+		WARN_ON(atomic_read(&sk->sk_refcnt) == 1);
+		__sock_put(sk);
+	}
+	return rc;
+}
+
 static __inline__ void __sk_add_node(struct sock *sk, struct hlist_head *list)
 {
 	hlist_add_head(&sk->sk_node, list);
@@ -372,6 +406,18 @@ static __inline__ void sk_add_node(struct sock *sk, struct hlist_head *list)
 	__sk_add_node(sk, list);
 }
 
+static inline void __sk_add_node_rcu(struct sock *sk,
+					 struct hlist_head *list)
+{
+	hlist_add_head_rcu(&sk->sk_node, list);
+}
+
+static inline void sk_add_node_rcu(struct sock *sk, struct hlist_head *list)
+{
+	sock_hold(sk);
+	__sk_add_node_rcu(sk, list);
+}
+
 static __inline__ void __sk_del_bind_node(struct sock *sk)
 {
 	__hlist_del(&sk->sk_bind_node);
@@ -385,9 +431,14 @@ static __inline__ void sk_add_bind_node(struct sock *sk,
 
 #define sk_for_each(__sk, node, list) \
 	hlist_for_each_entry(__sk, node, list, sk_node)
+#define sk_for_each_rcu(__sk, node, list) \
+	hlist_for_each_entry_rcu(__sk, node, list, sk_node)
 #define sk_for_each_from(__sk, node) \
 	if (__sk && ({ node = &(__sk)->sk_node; 1; })) \
 		hlist_for_each_entry_from(__sk, node, sk_node)
+#define sk_for_each_from_rcu(__sk, node) \
+	if (__sk && ({ node = &(__sk)->sk_node; 1; })) \
+		hlist_for_each_entry_from_rcu(__sk, node, sk_node)
 #define sk_for_each_continue(__sk, node) \
 	if (__sk && ({ node = &(__sk)->sk_node; 1; })) \
 		hlist_for_each_entry_continue(__sk, node, sk_node)
diff --git a/include/net/udp.h b/include/net/udp.h
index addcdc6..e97664b 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -51,7 +51,7 @@ struct udp_skb_cb {
 #define UDP_SKB_CB(__skb)	((struct udp_skb_cb *)((__skb)->cb))
 
 extern struct hlist_head udp_hash[UDP_HTABLE_SIZE];
-extern rwlock_t udp_hash_lock;
+extern spinlock_t udp_hash_wlock;
 
 
 /* Note: this must match 'valbool' in sock_setsockopt */
@@ -112,12 +112,13 @@ static inline void udp_lib_hash(struct sock *sk)
 
 static inline void udp_lib_unhash(struct sock *sk)
 {
-	write_lock_bh(&udp_hash_lock);
-	if (sk_del_node_init(sk)) {
+	spin_lock_bh(&udp_hash_wlock);
+	if (sk_del_node_rcu(sk)) {
 		inet_sk(sk)->num = 0;
 		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
 	}
-	write_unlock_bh(&udp_hash_lock);
+	spin_unlock_bh(&udp_hash_wlock);
+	synchronize_rcu();
 }
 
 static inline void udp_lib_close(struct sock *sk, long timeout)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 57e26fa..082e075 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -112,7 +112,8 @@ DEFINE_SNMP_STAT(struct udp_mib, udp_stats_in6) __read_mostly;
 EXPORT_SYMBOL(udp_stats_in6);
 
 struct hlist_head udp_hash[UDP_HTABLE_SIZE];
-DEFINE_RWLOCK(udp_hash_lock);
+DEFINE_SPINLOCK(udp_hash_wlock);
+EXPORT_SYMBOL(udp_hash_wlock);
 
 int sysctl_udp_mem[3] __read_mostly;
 int sysctl_udp_rmem_min __read_mostly;
@@ -155,7 +156,7 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
 	int    error = 1;
 	struct net *net = sock_net(sk);
 
-	write_lock_bh(&udp_hash_lock);
+	spin_lock_bh(&udp_hash_wlock);
 
 	if (!snum) {
 		int i, low, high, remaining;
@@ -225,12 +226,12 @@ gotit:
 	sk->sk_hash = snum;
 	if (sk_unhashed(sk)) {
 		head = &udptable[udp_hashfn(net, snum)];
-		sk_add_node(sk, head);
+		sk_add_node_rcu(sk, head);
 		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 	}
 	error = 0;
 fail:
-	write_unlock_bh(&udp_hash_lock);
+	spin_unlock_bh(&udp_hash_wlock);
 	return error;
 }
 
@@ -260,8 +261,8 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
 	unsigned short hnum = ntohs(dport);
 	int badness = -1;
 
-	read_lock(&udp_hash_lock);
-	sk_for_each(sk, node, &udptable[udp_hashfn(net, hnum)]) {
+	rcu_read_lock();
+	sk_for_each_rcu(sk, node, &udptable[udp_hashfn(net, hnum)]) {
 		struct inet_sock *inet = inet_sk(sk);
 
 		if (net_eq(sock_net(sk), net) && sk->sk_hash == hnum &&
@@ -296,9 +297,17 @@ static struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
 			}
 		}
 	}
+	/*
+	 * Note that this is safe, even with an RCU lock.
+	 * udp_lib_unhash() is the removal function, it calls
+	 * synchronize_sched() and the socket counter cannot go to
+	 * zero until it returns.  So if we increment it inside the
+	 * RCU read lock, it should never go to zero and then be
+	 * incremented again.
+	 */
 	if (result)
 		sock_hold(result);
-	read_unlock(&udp_hash_lock);
+	rcu_read_unlock();
 	return result;
 }
 
@@ -311,7 +320,7 @@ static inline struct sock *udp_v4_mcast_next(struct sock *sk,
 	struct sock *s = sk;
 	unsigned short hnum = ntohs(loc_port);
 
-	sk_for_each_from(s, node) {
+	sk_for_each_from_rcu(s, node) {
 		struct inet_sock *inet = inet_sk(s);
 
 		if (s->sk_hash != hnum					||
@@ -1094,8 +1103,8 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	struct sock *sk;
 	int dif;
 
-	read_lock(&udp_hash_lock);
-	sk = sk_head(&udptable[udp_hashfn(net, ntohs(uh->dest))]);
+	rcu_read_lock();
+	sk = sk_head_rcu(&udptable[udp_hashfn(net, ntohs(uh->dest))]);
 	dif = skb->dev->ifindex;
 	sk = udp_v4_mcast_next(sk, uh->dest, daddr, uh->source, saddr, dif);
 	if (sk) {
@@ -1104,8 +1113,9 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 		do {
 			struct sk_buff *skb1 = skb;
 
-			sknext = udp_v4_mcast_next(sk_next(sk), uh->dest, daddr,
-						   uh->source, saddr, dif);
+			sknext = udp_v4_mcast_next(sk_next_rcu(sk), uh->dest,
+						   daddr, uh->source, saddr,
+						   dif);
 			if (sknext)
 				skb1 = skb_clone(skb, GFP_ATOMIC);
 
@@ -1120,7 +1130,7 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 		} while (sknext);
 	} else
 		kfree_skb(skb);
-	read_unlock(&udp_hash_lock);
+	rcu_read_unlock();
 	return 0;
 }
 
@@ -1543,13 +1553,13 @@ static struct sock *udp_get_next(struct seq_file *seq, struct sock *sk)
 	struct net *net = seq_file_net(seq);
 
 	do {
-		sk = sk_next(sk);
+		sk = sk_next_rcu(sk);
 try_again:
 		;
 	} while (sk && (!net_eq(sock_net(sk), net) || sk->sk_family != state->family));
 
 	if (!sk && ++state->bucket < UDP_HTABLE_SIZE) {
-		sk = sk_head(state->hashtable + state->bucket);
+		sk = sk_head_rcu(state->hashtable + state->bucket);
 		goto try_again;
 	}
 	return sk;
@@ -1566,9 +1576,8 @@ static struct sock *udp_get_idx(struct seq_file *seq, loff_t pos)
 }
 
 static void *udp_seq_start(struct seq_file *seq, loff_t *pos)
-	__acquires(udp_hash_lock)
 {
-	read_lock(&udp_hash_lock);
+	rcu_read_lock();
 	return *pos ? udp_get_idx(seq, *pos-1) : SEQ_START_TOKEN;
 }
 
@@ -1586,9 +1595,8 @@ static void *udp_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 }
 
 static void udp_seq_stop(struct seq_file *seq, void *v)
-	__releases(udp_hash_lock)
 {
-	read_unlock(&udp_hash_lock);
+	rcu_read_unlock();
 }
 
 static int udp_seq_open(struct inode *inode, struct file *file)
@@ -1732,7 +1740,6 @@ void __init udp_init(void)
 
 EXPORT_SYMBOL(udp_disconnect);
 EXPORT_SYMBOL(udp_hash);
-EXPORT_SYMBOL(udp_hash_lock);
 EXPORT_SYMBOL(udp_ioctl);
 EXPORT_SYMBOL(udp_prot);
 EXPORT_SYMBOL(udp_sendmsg);
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index a6aecf7..b807de7 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -64,8 +64,8 @@ static struct sock *__udp6_lib_lookup(struct net *net,
 	unsigned short hnum = ntohs(dport);
 	int badness = -1;
 
-	read_lock(&udp_hash_lock);
-	sk_for_each(sk, node, &udptable[udp_hashfn(net, hnum)]) {
+	rcu_read_lock();
+	sk_for_each_rcu(sk, node, &udptable[udp_hashfn(net, hnum)]) {
 		struct inet_sock *inet = inet_sk(sk);
 
 		if (net_eq(sock_net(sk), net) && sk->sk_hash == hnum &&
@@ -101,9 +101,10 @@ static struct sock *__udp6_lib_lookup(struct net *net,
 			}
 		}
 	}
+	/* See comment in __udp4_lib_lookup on why this is safe. */
 	if (result)
 		sock_hold(result);
-	read_unlock(&udp_hash_lock);
+	rcu_read_unlock();
 	return result;
 }
 
@@ -322,7 +323,7 @@ static struct sock *udp_v6_mcast_next(struct sock *sk,
 	struct sock *s = sk;
 	unsigned short num = ntohs(loc_port);
 
-	sk_for_each_from(s, node) {
+	sk_for_each_from_rcu(s, node) {
 		struct inet_sock *inet = inet_sk(s);
 
 		if (sock_net(s) != sock_net(sk))
@@ -365,8 +366,8 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	const struct udphdr *uh = udp_hdr(skb);
 	int dif;
 
-	read_lock(&udp_hash_lock);
-	sk = sk_head(&udptable[udp_hashfn(net, ntohs(uh->dest))]);
+	rcu_read_lock();
+	sk = sk_head_rcu(&udptable[udp_hashfn(net, ntohs(uh->dest))]);
 	dif = inet6_iif(skb);
 	sk = udp_v6_mcast_next(sk, uh->dest, daddr, uh->source, saddr, dif);
 	if (!sk) {
@@ -375,7 +376,7 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	}
 
 	sk2 = sk;
-	while ((sk2 = udp_v6_mcast_next(sk_next(sk2), uh->dest, daddr,
+	while ((sk2 = udp_v6_mcast_next(sk_next_rcu(sk2), uh->dest, daddr,
 					uh->source, saddr, dif))) {
 		struct sk_buff *buff = skb_clone(skb, GFP_ATOMIC);
 		if (buff) {
@@ -394,7 +395,7 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 		sk_add_backlog(sk, skb);
 	bh_unlock_sock(sk);
 out:
-	read_unlock(&udp_hash_lock);
+	rcu_read_unlock();
 	return 0;
 }
 

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