[PATCH net-next v2] net: core: decouple ifalias get/set from rtnl lock

[PATCH net-next v2] net: core: decouple ifalias get/set from rtnl lock

[Florian Westphal]

Device alias can be set by either rtnetlink (rtnl is held) or sysfs.

rtnetlink hold the rtnl mutex, sysfs acquires it for this purpose.
Add an extra mutex for it plus a seqcount to get a consistent snapshot
of the alias buffer.

This allows the sysfs path to not take rtnl and would later allow
to not hold it when dumping ifalias.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Changes since v1:
 - add a comment why dev->ifalias is freed
 via kfree, not kfree_rcu.

 include/linux/netdevice.h |  3 +-
 net/core/dev.c            | 70 +++++++++++++++++++++++++++++++++++++++--------
 net/core/net-sysfs.c      | 17 ++++++------
 net/core/rtnetlink.c      | 13 +++++++--
 4 files changed, 80 insertions(+), 23 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index f535779d9dc1..5e03acf189e6 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1632,7 +1632,7 @@ enum netdev_priv_flags {
 struct net_device {
 	char			name[IFNAMSIZ];
 	struct hlist_node	name_hlist;
-	char 			*ifalias;
+	char			__rcu *ifalias;
 	/*
 	 *	I/O specific fields
 	 *	FIXME: Merge these and struct ifmap into one
@@ -3275,6 +3275,7 @@ void __dev_notify_flags(struct net_device *, unsigned int old_flags,
 			unsigned int gchanges);
 int dev_change_name(struct net_device *, const char *);
 int dev_set_alias(struct net_device *, const char *, size_t);
+int dev_get_alias(const struct net_device *, char *, size_t);
 int dev_change_net_namespace(struct net_device *, struct net *, const char *);
 int __dev_set_mtu(struct net_device *, int);
 int dev_set_mtu(struct net_device *, int);
diff --git a/net/core/dev.c b/net/core/dev.c
index e350c768d4b5..a7ac2902d702 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -188,6 +188,9 @@ static struct napi_struct *napi_by_id(unsigned int napi_id);
 DEFINE_RWLOCK(dev_base_lock);
 EXPORT_SYMBOL(dev_base_lock);
 
+static DEFINE_MUTEX(ifalias_mutex);
+static seqcount_t ifalias_rename_seq;
+
 /* protects napi_hash addition/deletion and napi_gen_id */
 static DEFINE_SPINLOCK(napi_hash_lock);
 
@@ -1265,29 +1268,72 @@ int dev_change_name(struct net_device *dev, const char *newname)
  */
 int dev_set_alias(struct net_device *dev, const char *alias, size_t len)
 {
-	char *new_ifalias;
-
-	ASSERT_RTNL();
+	char *new_ifalias, *old_ifalias;
 
 	if (len >= IFALIASZ)
 		return -EINVAL;
 
+	mutex_lock(&ifalias_mutex);
+
+	old_ifalias = rcu_dereference_protected(dev->ifalias,
+						mutex_is_locked(&ifalias_mutex));
 	if (!len) {
-		kfree(dev->ifalias);
-		dev->ifalias = NULL;
-		return 0;
+		RCU_INIT_POINTER(dev->ifalias, NULL);
+		goto out;
 	}
 
-	new_ifalias = krealloc(dev->ifalias, len + 1, GFP_KERNEL);
-	if (!new_ifalias)
+	new_ifalias = __krealloc(old_ifalias, len + 1, GFP_KERNEL);
+	if (!new_ifalias) {
+		mutex_unlock(&ifalias_mutex);
 		return -ENOMEM;
-	dev->ifalias = new_ifalias;
-	memcpy(dev->ifalias, alias, len);
-	dev->ifalias[len] = 0;
+	}
 
+	if (new_ifalias == old_ifalias) {
+		write_seqcount_begin(&ifalias_rename_seq);
+		memcpy(new_ifalias, alias, len);
+		new_ifalias[len] = 0;
+		write_seqcount_end(&ifalias_rename_seq);
+		mutex_unlock(&ifalias_mutex);
+		return len;
+	}
+
+	memcpy(new_ifalias, alias, len);
+	new_ifalias[len] = 0;
+
+	rcu_assign_pointer(dev->ifalias, new_ifalias);
+out:
+	mutex_unlock(&ifalias_mutex);
+	if (old_ifalias) {
+		synchronize_net();
+		kfree(old_ifalias);
+	}
 	return len;
 }
 
+int dev_get_alias(const struct net_device *dev, char *alias, size_t len)
+{
+	unsigned int seq;
+	int ret;
+
+	for (;;) {
+		const char *name;
+
+		ret = 0;
+		rcu_read_lock();
+		name = rcu_dereference(dev->ifalias);
+		seq = raw_seqcount_begin(&ifalias_rename_seq);
+		if (name)
+			ret = snprintf(alias, len, "%s", name);
+		rcu_read_unlock();
+
+		if (!read_seqcount_retry(&ifalias_rename_seq, seq))
+			break;
+
+		cond_resched();
+	}
+
+	return ret;
+}
 
 /**
  *	netdev_features_change - device changes features
@@ -8770,6 +8816,8 @@ static int __init net_dev_init(void)
 				       NULL, dev_cpu_dead);
 	WARN_ON(rc < 0);
 	rc = 0;
+
+	seqcount_init(&ifalias_rename_seq);
 out:
 	return rc;
 }
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 927a6dcbad96..51d5836d8fb9 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -391,10 +391,7 @@ static ssize_t ifalias_store(struct device *dev, struct device_attribute *attr,
 	if (len >  0 && buf[len - 1] == '\n')
 		--count;
 
-	if (!rtnl_trylock())
-		return restart_syscall();
 	ret = dev_set_alias(netdev, buf, count);
-	rtnl_unlock();
 
 	return ret < 0 ? ret : len;
 }
@@ -403,13 +400,12 @@ static ssize_t ifalias_show(struct device *dev,
 			    struct device_attribute *attr, char *buf)
 {
 	const struct net_device *netdev = to_net_dev(dev);
+	char tmp[IFALIASZ];
 	ssize_t ret = 0;
 
-	if (!rtnl_trylock())
-		return restart_syscall();
-	if (netdev->ifalias)
-		ret = sprintf(buf, "%s\n", netdev->ifalias);
-	rtnl_unlock();
+	ret = dev_get_alias(netdev, tmp, sizeof(tmp));
+	if (ret > 0)
+		ret = sprintf(buf, "%s\n", tmp);
 	return ret;
 }
 static DEVICE_ATTR_RW(ifalias);
@@ -1488,7 +1484,10 @@ static void netdev_release(struct device *d)
 
 	BUG_ON(dev->reg_state != NETREG_RELEASED);
 
-	kfree(dev->ifalias);
+	/* no need to wait for rcu grace period:
+	 * device is dead and about to be freed.
+	 */
+	kfree(rcu_access_pointer(dev->ifalias));
 	netdev_freemem(dev);
 }
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index e6955da0d58d..3961f87cdc76 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1366,6 +1366,16 @@ static int nla_put_iflink(struct sk_buff *skb, const struct net_device *dev)
 	return nla_put_u32(skb, IFLA_LINK, ifindex);
 }
 
+static noinline_for_stack int nla_put_ifalias(struct sk_buff *skb,
+					      struct net_device *dev)
+{
+	char buf[IFALIASZ];
+	int ret;
+
+	ret = dev_get_alias(dev, buf, sizeof(buf));
+	return ret > 0 ? nla_put_string(skb, IFLA_IFALIAS, buf) : 0;
+}
+
 static int rtnl_fill_link_netnsid(struct sk_buff *skb,
 				  const struct net_device *dev)
 {
@@ -1425,8 +1435,7 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb, struct net_device *dev,
 	    nla_put_u8(skb, IFLA_CARRIER, netif_carrier_ok(dev)) ||
 	    (dev->qdisc &&
 	     nla_put_string(skb, IFLA_QDISC, dev->qdisc->ops->id)) ||
-	    (dev->ifalias &&
-	     nla_put_string(skb, IFLA_IFALIAS, dev->ifalias)) ||
+	    nla_put_ifalias(skb, dev) ||
 	    nla_put_u32(skb, IFLA_CARRIER_CHANGES,
 			atomic_read(&dev->carrier_changes)) ||
 	    nla_put_u8(skb, IFLA_PROTO_DOWN, dev->proto_down))
-- 
2.13.6

Re: [PATCH net-next v2] net: core: decouple ifalias get/set from rtnl lock

[Eric Dumazet]

On Mon, 2017-10-02 at 12:27 +0200, Florian Westphal wrote:
> Device alias can be set by either rtnetlink (rtnl is held) or sysfs.
> 
> rtnetlink hold the rtnl mutex, sysfs acquires it for this purpose.
> Add an extra mutex for it plus a seqcount to get a consistent snapshot
> of the alias buffer.


> +int dev_get_alias(const struct net_device *dev, char *alias, size_t len)
> +{
> +	unsigned int seq;
> +	int ret;
> +
> +	for (;;) {
> +		const char *name;
> +
> +		ret = 0;
> +		rcu_read_lock();
> +		name = rcu_dereference(dev->ifalias);
> +		seq = raw_seqcount_begin(&ifalias_rename_seq);
> +		if (name)
> +			ret = snprintf(alias, len, "%s", name);
> +		rcu_read_unlock();
> +
> +		if (!read_seqcount_retry(&ifalias_rename_seq, seq))
> +			break;
> +
> +		cond_resched();
> +	}
> +
> +	return ret;
> +}

I believe this too complex and not needed.

Just use RCU : A writer is supposed to work on a private copy, and
_then_ publish the new pointer, so that a reader can not see mangled
string.

We either copy the 'old' name or the 'new' one.

A seqcount is not needed, and wont prevent you from reading the value
right before a change anyway.

Re: [PATCH net-next v2] net: core: decouple ifalias get/set from rtnl lock

[Florian Westphal]

Eric Dumazet <eric.dumazet@gmail.com> wrote:
> Just use RCU : A writer is supposed to work on a private copy, and
> _then_ publish the new pointer, so that a reader can not see mangled
> string.
> 
> We either copy the 'old' name or the 'new' one.
> 
> A seqcount is not needed, and wont prevent you from reading the value
> right before a change anyway.

Would you rather use kfree_rcu or unconditional synchronize_net()
before releasing old memory?

Re: [PATCH net-next v2] net: core: decouple ifalias get/set from rtnl lock

[Eric Dumazet]

On Mon, 2017-10-02 at 17:09 +0200, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > Just use RCU : A writer is supposed to work on a private copy, and
> > _then_ publish the new pointer, so that a reader can not see mangled
> > string.
> > 
> > We either copy the 'old' name or the 'new' one.
> > 
> > A seqcount is not needed, and wont prevent you from reading the value
> > right before a change anyway.
> 
> Would you rather use kfree_rcu or unconditional synchronize_net()
> before releasing old memory?

kfree_rcu() please ;)

Adding 16 bytes for the rcu_head is acceptable I think.