net/ipv4/tcp_cong.c - arm/linux - Git at Google

 /*
  * Pluggable TCP congestion control support and newReno
  * congestion control.
  * Based on ideas from I/O scheduler support and Web100.
  *
  * Copyright (C) 2005 Stephen Hemminger <shemminger@osdl.org>
  */

 #define pr_fmt(fmt) "TCP: " fmt

 #include <linux/module.h>
 #include <linux/mm.h>
 #include <linux/types.h>
 #include <linux/list.h>
 #include <linux/gfp.h>
 #include <linux/jhash.h>
 #include <net/tcp.h>

 static DEFINE_SPINLOCK(tcp_cong_list_lock);
 static LIST_HEAD(tcp_cong_list);

 /* Simple linear search, don't expect many entries! */
 static struct tcp_congestion_ops *tcp_ca_find(const char *name)
 {
 	struct tcp_congestion_ops *e;

 	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
 		if (strcmp(e->name, name) == 0)
 			return e;
 	}

 	return NULL;
 }

 /* Must be called with rcu lock held */
 static const struct tcp_congestion_ops *__tcp_ca_find_autoload(const char *name)
 {
 	const struct tcp_congestion_ops *ca = tcp_ca_find(name);
 #ifdef CONFIG_MODULES
 	if (!ca && capable(CAP_NET_ADMIN)) {
 		rcu_read_unlock();
 		request_module("tcp_%s", name);
 		rcu_read_lock();
 		ca = tcp_ca_find(name);
 	}
 #endif
 	return ca;
 }

 /* Simple linear search, not much in here. */
 struct tcp_congestion_ops *tcp_ca_find_key(u32 key)
 {
 	struct tcp_congestion_ops *e;

 	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
 		if (e->key == key)
 			return e;
 	}

 	return NULL;
 }

 /*
  * Attach new congestion control algorithm to the list
  * of available options.
  */
 int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
 {
 	int ret = 0;

 	/* all algorithms must implement these */
 	if (!ca->ssthresh || !ca->undo_cwnd ||
 	    !(ca->cong_avoid || ca->cong_control)) {
 		pr_err("%s does not implement required ops\n", ca->name);
 		return -EINVAL;
 	}

 	ca->key = jhash(ca->name, sizeof(ca->name), strlen(ca->name));

 	spin_lock(&tcp_cong_list_lock);
 	if (ca->key == TCP_CA_UNSPEC || tcp_ca_find_key(ca->key)) {
 		pr_notice("%s already registered or non-unique key\n",
 			  ca->name);
 		ret = -EEXIST;
 	} else {
 		list_add_tail_rcu(&ca->list, &tcp_cong_list);
 		pr_debug("%s registered\n", ca->name);
 	}
 	spin_unlock(&tcp_cong_list_lock);

 	return ret;
 }
 EXPORT_SYMBOL_GPL(tcp_register_congestion_control);

 /*
  * Remove congestion control algorithm, called from
  * the module's remove function.  Module ref counts are used
  * to ensure that this can't be done till all sockets using
  * that method are closed.
  */
 void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
 {
 	spin_lock(&tcp_cong_list_lock);
 	list_del_rcu(&ca->list);
 	spin_unlock(&tcp_cong_list_lock);

 	/* Wait for outstanding readers to complete before the
 	 * module gets removed entirely.
 	 *
 	 * A try_module_get() should fail by now as our module is
 	 * in "going" state since no refs are held anymore and
 	 * module_exit() handler being called.
 	 */
 	synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);

 u32 tcp_ca_get_key_by_name(const char *name, bool *ecn_ca)
 {
 	const struct tcp_congestion_ops *ca;
 	u32 key = TCP_CA_UNSPEC;

 	might_sleep();

 	rcu_read_lock();
 	ca = __tcp_ca_find_autoload(name);
 	if (ca) {
 		key = ca->key;
 		*ecn_ca = ca->flags & TCP_CONG_NEEDS_ECN;
 	}
 	rcu_read_unlock();

 	return key;
 }
 EXPORT_SYMBOL_GPL(tcp_ca_get_key_by_name);

 char *tcp_ca_get_name_by_key(u32 key, char *buffer)
 {
 	const struct tcp_congestion_ops *ca;
 	char *ret = NULL;

 	rcu_read_lock();
 	ca = tcp_ca_find_key(key);
 	if (ca)
 		ret = strncpy(buffer, ca->name,
 			      TCP_CA_NAME_MAX);
 	rcu_read_unlock();

 	return ret;
 }
 EXPORT_SYMBOL_GPL(tcp_ca_get_name_by_key);

 /* Assign choice of congestion control. */
 void tcp_assign_congestion_control(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_congestion_ops *ca;

 	rcu_read_lock();
 	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
 		if (likely(try_module_get(ca->owner))) {
 			icsk->icsk_ca_ops = ca;
 			goto out;
 		}
 		/* Fallback to next available. The last really
 		 * guaranteed fallback is Reno from this list.
 		 */
 	}
 out:
 	rcu_read_unlock();
 	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));

 	if (ca->flags & TCP_CONG_NEEDS_ECN)
 		INET_ECN_xmit(sk);
 	else
 		INET_ECN_dontxmit(sk);
 }

 void tcp_init_congestion_control(struct sock *sk)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);

 	tcp_sk(sk)->prior_ssthresh = 0;
 	if (icsk->icsk_ca_ops->init)
 		icsk->icsk_ca_ops->init(sk);
 	if (tcp_ca_needs_ecn(sk))
 		INET_ECN_xmit(sk);
 	else
 		INET_ECN_dontxmit(sk);
 }

 static void tcp_reinit_congestion_control(struct sock *sk,
 					  const struct tcp_congestion_ops *ca)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);

 	tcp_cleanup_congestion_control(sk);
 	icsk->icsk_ca_ops = ca;
 	icsk->icsk_ca_setsockopt = 1;
 	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));

 	if (sk->sk_state != TCP_CLOSE)
 		tcp_init_congestion_control(sk);
 }

 /* Manage refcounts on socket close. */
 void tcp_cleanup_congestion_control(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);

 	if (icsk->icsk_ca_ops->release)
 		icsk->icsk_ca_ops->release(sk);
 	module_put(icsk->icsk_ca_ops->owner);
 }

 /* Used by sysctl to change default congestion control */
 int tcp_set_default_congestion_control(const char *name)
 {
 	struct tcp_congestion_ops *ca;
 	int ret = -ENOENT;

 	spin_lock(&tcp_cong_list_lock);
 	ca = tcp_ca_find(name);
 #ifdef CONFIG_MODULES
 	if (!ca && capable(CAP_NET_ADMIN)) {
 		spin_unlock(&tcp_cong_list_lock);

 		request_module("tcp_%s", name);
 		spin_lock(&tcp_cong_list_lock);
 		ca = tcp_ca_find(name);
 	}
 #endif

 	if (ca) {
 		ca->flags |= TCP_CONG_NON_RESTRICTED;	/* default is always allowed */
 		list_move(&ca->list, &tcp_cong_list);
 		ret = 0;
 	}
 	spin_unlock(&tcp_cong_list_lock);

 	return ret;
 }

 /* Set default value from kernel configuration at bootup */
 static int __init tcp_congestion_default(void)
 {
 	return tcp_set_default_congestion_control(CONFIG_DEFAULT_TCP_CONG);
 }
 late_initcall(tcp_congestion_default);

 /* Build string with list of available congestion control values */
 void tcp_get_available_congestion_control(char *buf, size_t maxlen)
 {
 	struct tcp_congestion_ops *ca;
 	size_t offs = 0;

 	rcu_read_lock();
 	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
 		offs += snprintf(buf + offs, maxlen - offs,
 				 "%s%s",
 				 offs == 0 ? "" : " ", ca->name);
 	}
 	rcu_read_unlock();
 }

 /* Get current default congestion control */
 void tcp_get_default_congestion_control(char *name)
 {
 	struct tcp_congestion_ops *ca;
 	/* We will always have reno... */
 	BUG_ON(list_empty(&tcp_cong_list));

 	rcu_read_lock();
 	ca = list_entry(tcp_cong_list.next, struct tcp_congestion_ops, list);
 	strncpy(name, ca->name, TCP_CA_NAME_MAX);
 	rcu_read_unlock();
 }

 /* Built list of non-restricted congestion control values */
 void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
 {
 	struct tcp_congestion_ops *ca;
 	size_t offs = 0;

 	*buf = '\0';
 	rcu_read_lock();
 	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
 		if (!(ca->flags & TCP_CONG_NON_RESTRICTED))
 			continue;
 		offs += snprintf(buf + offs, maxlen - offs,
 				 "%s%s",
 				 offs == 0 ? "" : " ", ca->name);
 	}
 	rcu_read_unlock();
 }

 /* Change list of non-restricted congestion control */
 int tcp_set_allowed_congestion_control(char *val)
 {
 	struct tcp_congestion_ops *ca;
 	char *saved_clone, *clone, *name;
 	int ret = 0;

 	saved_clone = clone = kstrdup(val, GFP_USER);
 	if (!clone)
 		return -ENOMEM;

 	spin_lock(&tcp_cong_list_lock);
 	/* pass 1 check for bad entries */
 	while ((name = strsep(&clone, " ")) && *name) {
 		ca = tcp_ca_find(name);
 		if (!ca) {
 			ret = -ENOENT;
 			goto out;
 		}
 	}

 	/* pass 2 clear old values */
 	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
 		ca->flags &= ~TCP_CONG_NON_RESTRICTED;

 	/* pass 3 mark as allowed */
 	while ((name = strsep(&val, " ")) && *name) {
 		ca = tcp_ca_find(name);
 		WARN_ON(!ca);
 		if (ca)
 			ca->flags |= TCP_CONG_NON_RESTRICTED;
 	}
 out:
 	spin_unlock(&tcp_cong_list_lock);
 	kfree(saved_clone);

 	return ret;
 }

 /* Change congestion control for socket. If load is false, then it is the
  * responsibility of the caller to call tcp_init_congestion_control or
  * tcp_reinit_congestion_control (if the current congestion control was
  * already initialized.
  */
 int tcp_set_congestion_control(struct sock *sk, const char *name, bool load, bool reinit)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	const struct tcp_congestion_ops *ca;
 	int err = 0;

 	if (icsk->icsk_ca_dst_locked)
 		return -EPERM;

 	rcu_read_lock();
 	if (!load)
 		ca = tcp_ca_find(name);
 	else
 		ca = __tcp_ca_find_autoload(name);
 	/* No change asking for existing value */
 	if (ca == icsk->icsk_ca_ops) {
 		icsk->icsk_ca_setsockopt = 1;
 		goto out;
 	}
 	if (!ca) {
 		err = -ENOENT;
 	} else if (!load) {
 		const struct tcp_congestion_ops *old_ca = icsk->icsk_ca_ops;

 		if (try_module_get(ca->owner)) {
 			if (reinit) {
 				tcp_reinit_congestion_control(sk, ca);
 			} else {
 				icsk->icsk_ca_ops = ca;
 				module_put(old_ca->owner);
 			}
 		} else {
 			err = -EBUSY;
 		}
 	} else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) ||
 		     ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))) {
 		err = -EPERM;
 	} else if (!try_module_get(ca->owner)) {
 		err = -EBUSY;
 	} else {
 		tcp_reinit_congestion_control(sk, ca);
 	}
  out:
 	rcu_read_unlock();
 	return err;
 }

 /* Slow start is used when congestion window is no greater than the slow start
  * threshold. We base on RFC2581 and also handle stretch ACKs properly.
  * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
  * something better;) a packet is only considered (s)acked in its entirety to
  * defend the ACK attacks described in the RFC. Slow start processes a stretch
  * ACK of degree N as if N acks of degree 1 are received back to back except
  * ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
  * returns the leftover acks to adjust cwnd in congestion avoidance mode.
  */
 u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
 {
 	u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);

 	acked -= cwnd - tp->snd_cwnd;
 	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);

 	return acked;
 }
 EXPORT_SYMBOL_GPL(tcp_slow_start);

 /* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
  * for every packet that was ACKed.
  */
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
 {
 	/* If credits accumulated at a higher w, apply them gently now. */
 	if (tp->snd_cwnd_cnt >= w) {
 		tp->snd_cwnd_cnt = 0;
 		tp->snd_cwnd++;
 	}

 	tp->snd_cwnd_cnt += acked;
 	if (tp->snd_cwnd_cnt >= w) {
 		u32 delta = tp->snd_cwnd_cnt / w;

 		tp->snd_cwnd_cnt -= delta * w;
 		tp->snd_cwnd += delta;
 	}
 	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
 }
 EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

 /*
  * TCP Reno congestion control
  * This is special case used for fallback as well.
  */
 /* This is Jacobson's slow start and congestion avoidance.
  * SIGCOMM '88, p. 328.
  */
 void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 {
 	struct tcp_sock *tp = tcp_sk(sk);

 	if (!tcp_is_cwnd_limited(sk))
 		return;

 	/* In "safe" area, increase. */
 	if (tcp_in_slow_start(tp)) {
 		acked = tcp_slow_start(tp, acked);
 		if (!acked)
 			return;
 	}
 	/* In dangerous area, increase slowly. */
 	tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);

 /* Slow start threshold is half the congestion window (min 2) */
 u32 tcp_reno_ssthresh(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);

 	return max(tp->snd_cwnd >> 1U, 2U);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);

 u32 tcp_reno_undo_cwnd(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);

 	return max(tp->snd_cwnd, tp->prior_cwnd);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);

 struct tcp_congestion_ops tcp_reno = {
 	.flags		= TCP_CONG_NON_RESTRICTED,
 	.name		= "reno",
 	.owner		= THIS_MODULE,
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_reno_cong_avoid,
 	.undo_cwnd	= tcp_reno_undo_cwnd,
 };
	/*
	* Pluggable TCP congestion control support and newReno
	* congestion control.
	* Based on ideas from I/O scheduler support and Web100.
	*
	* Copyright (C) 2005 Stephen Hemminger <shemminger@osdl.org>
	*/

	#define pr_fmt(fmt) "TCP: " fmt

	#include <linux/module.h>
	#include <linux/mm.h>
	#include <linux/types.h>
	#include <linux/list.h>
	#include <linux/gfp.h>
	#include <linux/jhash.h>
	#include <net/tcp.h>

	static DEFINE_SPINLOCK(tcp_cong_list_lock);
	static LIST_HEAD(tcp_cong_list);

	/* Simple linear search, don't expect many entries! */
	static struct tcp_congestion_ops tcp_ca_find(const char name)
	{
	struct tcp_congestion_ops *e;

	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
	if (strcmp(e->name, name) == 0)
	return e;
	}

	return NULL;
	}

	/* Must be called with rcu lock held */
	static const struct tcp_congestion_ops __tcp_ca_find_autoload(const char name)
	{
	const struct tcp_congestion_ops *ca = tcp_ca_find(name);
	#ifdef CONFIG_MODULES
	if (!ca && capable(CAP_NET_ADMIN)) {
	rcu_read_unlock();
	request_module("tcp_%s", name);
	rcu_read_lock();
	ca = tcp_ca_find(name);
	}
	#endif
	return ca;
	}

	/* Simple linear search, not much in here. */
	struct tcp_congestion_ops *tcp_ca_find_key(u32 key)
	{
	struct tcp_congestion_ops *e;

	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
	if (e->key == key)
	return e;
	}

	return NULL;
	}

	/*
	* Attach new congestion control algorithm to the list
	* of available options.
	*/
	int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
	{
	int ret = 0;

	/* all algorithms must implement these */
	if (!ca->ssthresh \|\| !ca->undo_cwnd \|\|
	!(ca->cong_avoid \|\| ca->cong_control)) {
	pr_err("%s does not implement required ops\n", ca->name);
	return -EINVAL;
	}

	ca->key = jhash(ca->name, sizeof(ca->name), strlen(ca->name));

	spin_lock(&tcp_cong_list_lock);
	if (ca->key == TCP_CA_UNSPEC \|\| tcp_ca_find_key(ca->key)) {
	pr_notice("%s already registered or non-unique key\n",
	ca->name);
	ret = -EEXIST;
	} else {
	list_add_tail_rcu(&ca->list, &tcp_cong_list);
	pr_debug("%s registered\n", ca->name);
	}
	spin_unlock(&tcp_cong_list_lock);

	return ret;
	}
	EXPORT_SYMBOL_GPL(tcp_register_congestion_control);

	/*
	* Remove congestion control algorithm, called from
	* the module's remove function. Module ref counts are used
	* to ensure that this can't be done till all sockets using
	* that method are closed.
	*/
	void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
	{
	spin_lock(&tcp_cong_list_lock);
	list_del_rcu(&ca->list);
	spin_unlock(&tcp_cong_list_lock);

	/* Wait for outstanding readers to complete before the
	* module gets removed entirely.
	*
	* A try_module_get() should fail by now as our module is
	* in "going" state since no refs are held anymore and
	* module_exit() handler being called.
	*/
	synchronize_rcu();
	}
	EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);

	u32 tcp_ca_get_key_by_name(const char name, bool ecn_ca)
	{
	const struct tcp_congestion_ops *ca;
	u32 key = TCP_CA_UNSPEC;

	might_sleep();

	rcu_read_lock();
	ca = __tcp_ca_find_autoload(name);
	if (ca) {
	key = ca->key;
	*ecn_ca = ca->flags & TCP_CONG_NEEDS_ECN;
	}
	rcu_read_unlock();

	return key;
	}
	EXPORT_SYMBOL_GPL(tcp_ca_get_key_by_name);

	char tcp_ca_get_name_by_key(u32 key, char buffer)
	{
	const struct tcp_congestion_ops *ca;
	char *ret = NULL;

	rcu_read_lock();
	ca = tcp_ca_find_key(key);
	if (ca)
	ret = strncpy(buffer, ca->name,
	TCP_CA_NAME_MAX);
	rcu_read_unlock();

	return ret;
	}
	EXPORT_SYMBOL_GPL(tcp_ca_get_name_by_key);

	/* Assign choice of congestion control. */
	void tcp_assign_congestion_control(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_congestion_ops *ca;

	rcu_read_lock();
	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
	if (likely(try_module_get(ca->owner))) {
	icsk->icsk_ca_ops = ca;
	goto out;
	}
	/* Fallback to next available. The last really
	* guaranteed fallback is Reno from this list.
	*/
	}
	out:
	rcu_read_unlock();
	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));

	if (ca->flags & TCP_CONG_NEEDS_ECN)
	INET_ECN_xmit(sk);
	else
	INET_ECN_dontxmit(sk);
	}

	void tcp_init_congestion_control(struct sock *sk)
	{
	const struct inet_connection_sock *icsk = inet_csk(sk);

	tcp_sk(sk)->prior_ssthresh = 0;
	if (icsk->icsk_ca_ops->init)
	icsk->icsk_ca_ops->init(sk);
	if (tcp_ca_needs_ecn(sk))
	INET_ECN_xmit(sk);
	else
	INET_ECN_dontxmit(sk);
	}

	static void tcp_reinit_congestion_control(struct sock *sk,
	const struct tcp_congestion_ops *ca)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);

	tcp_cleanup_congestion_control(sk);
	icsk->icsk_ca_ops = ca;
	icsk->icsk_ca_setsockopt = 1;
	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));

	if (sk->sk_state != TCP_CLOSE)
	tcp_init_congestion_control(sk);
	}

	/* Manage refcounts on socket close. */
	void tcp_cleanup_congestion_control(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);

	if (icsk->icsk_ca_ops->release)
	icsk->icsk_ca_ops->release(sk);
	module_put(icsk->icsk_ca_ops->owner);
	}

	/* Used by sysctl to change default congestion control */
	int tcp_set_default_congestion_control(const char *name)
	{
	struct tcp_congestion_ops *ca;
	int ret = -ENOENT;

	spin_lock(&tcp_cong_list_lock);
	ca = tcp_ca_find(name);
	#ifdef CONFIG_MODULES
	if (!ca && capable(CAP_NET_ADMIN)) {
	spin_unlock(&tcp_cong_list_lock);

	request_module("tcp_%s", name);
	spin_lock(&tcp_cong_list_lock);
	ca = tcp_ca_find(name);
	}
	#endif

	if (ca) {
	ca->flags \|= TCP_CONG_NON_RESTRICTED; /* default is always allowed */
	list_move(&ca->list, &tcp_cong_list);
	ret = 0;
	}
	spin_unlock(&tcp_cong_list_lock);

	return ret;
	}

	/* Set default value from kernel configuration at bootup */
	static int __init tcp_congestion_default(void)
	{
	return tcp_set_default_congestion_control(CONFIG_DEFAULT_TCP_CONG);
	}
	late_initcall(tcp_congestion_default);

	/* Build string with list of available congestion control values */
	void tcp_get_available_congestion_control(char *buf, size_t maxlen)
	{
	struct tcp_congestion_ops *ca;
	size_t offs = 0;

	rcu_read_lock();
	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
	offs += snprintf(buf + offs, maxlen - offs,
	"%s%s",
	offs == 0 ? "" : " ", ca->name);
	}
	rcu_read_unlock();
	}

	/* Get current default congestion control */
	void tcp_get_default_congestion_control(char *name)
	{
	struct tcp_congestion_ops *ca;
	/* We will always have reno... */
	BUG_ON(list_empty(&tcp_cong_list));

	rcu_read_lock();
	ca = list_entry(tcp_cong_list.next, struct tcp_congestion_ops, list);
	strncpy(name, ca->name, TCP_CA_NAME_MAX);
	rcu_read_unlock();
	}

	/* Built list of non-restricted congestion control values */
	void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
	{
	struct tcp_congestion_ops *ca;
	size_t offs = 0;

	*buf = '\0';
	rcu_read_lock();
	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
	if (!(ca->flags & TCP_CONG_NON_RESTRICTED))
	continue;
	offs += snprintf(buf + offs, maxlen - offs,
	"%s%s",
	offs == 0 ? "" : " ", ca->name);
	}
	rcu_read_unlock();
	}

	/* Change list of non-restricted congestion control */
	int tcp_set_allowed_congestion_control(char *val)
	{
	struct tcp_congestion_ops *ca;
	char saved_clone, clone, *name;
	int ret = 0;

	saved_clone = clone = kstrdup(val, GFP_USER);
	if (!clone)
	return -ENOMEM;

	spin_lock(&tcp_cong_list_lock);
	/* pass 1 check for bad entries */
	while ((name = strsep(&clone, " ")) && *name) {
	ca = tcp_ca_find(name);
	if (!ca) {
	ret = -ENOENT;
	goto out;
	}
	}

	/* pass 2 clear old values */
	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
	ca->flags &= ~TCP_CONG_NON_RESTRICTED;

	/* pass 3 mark as allowed */
	while ((name = strsep(&val, " ")) && *name) {
	ca = tcp_ca_find(name);
	WARN_ON(!ca);
	if (ca)
	ca->flags \|= TCP_CONG_NON_RESTRICTED;
	}
	out:
	spin_unlock(&tcp_cong_list_lock);
	kfree(saved_clone);

	return ret;
	}

	/* Change congestion control for socket. If load is false, then it is the
	* responsibility of the caller to call tcp_init_congestion_control or
	* tcp_reinit_congestion_control (if the current congestion control was
	* already initialized.
	*/
	int tcp_set_congestion_control(struct sock sk, const char name, bool load, bool reinit)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	const struct tcp_congestion_ops *ca;
	int err = 0;

	if (icsk->icsk_ca_dst_locked)
	return -EPERM;

	rcu_read_lock();
	if (!load)
	ca = tcp_ca_find(name);
	else
	ca = __tcp_ca_find_autoload(name);
	/* No change asking for existing value */
	if (ca == icsk->icsk_ca_ops) {
	icsk->icsk_ca_setsockopt = 1;
	goto out;
	}
	if (!ca) {
	err = -ENOENT;
	} else if (!load) {
	const struct tcp_congestion_ops *old_ca = icsk->icsk_ca_ops;

	if (try_module_get(ca->owner)) {
	if (reinit) {
	tcp_reinit_congestion_control(sk, ca);
	} else {
	icsk->icsk_ca_ops = ca;
	module_put(old_ca->owner);
	}
	} else {
	err = -EBUSY;
	}
	} else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) \|\|
	ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))) {
	err = -EPERM;
	} else if (!try_module_get(ca->owner)) {
	err = -EBUSY;
	} else {
	tcp_reinit_congestion_control(sk, ca);
	}
	out:
	rcu_read_unlock();
	return err;
	}

	/* Slow start is used when congestion window is no greater than the slow start
	* threshold. We base on RFC2581 and also handle stretch ACKs properly.
	* We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
	* something better;) a packet is only considered (s)acked in its entirety to
	* defend the ACK attacks described in the RFC. Slow start processes a stretch
	* ACK of degree N as if N acks of degree 1 are received back to back except
	* ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
	* returns the leftover acks to adjust cwnd in congestion avoidance mode.
	*/
	u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
	{
	u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);

	acked -= cwnd - tp->snd_cwnd;
	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);

	return acked;
	}
	EXPORT_SYMBOL_GPL(tcp_slow_start);

	/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
	* for every packet that was ACKed.
	*/
	void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
	{
	/* If credits accumulated at a higher w, apply them gently now. */
	if (tp->snd_cwnd_cnt >= w) {
	tp->snd_cwnd_cnt = 0;
	tp->snd_cwnd++;
	}

	tp->snd_cwnd_cnt += acked;
	if (tp->snd_cwnd_cnt >= w) {
	u32 delta = tp->snd_cwnd_cnt / w;

	tp->snd_cwnd_cnt -= delta * w;
	tp->snd_cwnd += delta;
	}
	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
	}
	EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

	/*
	* TCP Reno congestion control
	* This is special case used for fallback as well.
	*/
	/* This is Jacobson's slow start and congestion avoidance.
	* SIGCOMM '88, p. 328.
	*/
	void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
	{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!tcp_is_cwnd_limited(sk))
	return;

	/* In "safe" area, increase. */
	if (tcp_in_slow_start(tp)) {
	acked = tcp_slow_start(tp, acked);
	if (!acked)
	return;
	}
	/* In dangerous area, increase slowly. */
	tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
	}
	EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);

	/* Slow start threshold is half the congestion window (min 2) */
	u32 tcp_reno_ssthresh(struct sock *sk)
	{
	const struct tcp_sock *tp = tcp_sk(sk);

	return max(tp->snd_cwnd >> 1U, 2U);
	}
	EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);

	u32 tcp_reno_undo_cwnd(struct sock *sk)
	{
	const struct tcp_sock *tp = tcp_sk(sk);

	return max(tp->snd_cwnd, tp->prior_cwnd);
	}
	EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);

	struct tcp_congestion_ops tcp_reno = {
	.flags = TCP_CONG_NON_RESTRICTED,
	.name = "reno",
	.owner = THIS_MODULE,
	.ssthresh = tcp_reno_ssthresh,
	.cong_avoid = tcp_reno_cong_avoid,
	.undo_cwnd = tcp_reno_undo_cwnd,
	};