net: add a noref bit on skb dst

Use low order bit of skb->_skb_dst to tell dst is not refcounted.

Change _skb_dst to _skb_refdst to make sure all uses are catched.

skb_dst() returns the dst, regardless of noref bit set or not, but
with a lockdep check to make sure a noref dst is not given if current
user is not rcu protected.

New skb_dst_set_noref() helper to set an notrefcounted dst on a skb.
(with lockdep check)

skb_dst_drop() drops a reference only if skb dst was refcounted.

skb_dst_force() helper is used to force a refcount on dst, when skb
is queued and not anymore RCU protected.

Use skb_dst_force() in __sk_add_backlog(), __dev_xmit_skb() if
!IFF_XMIT_DST_RELEASE or skb enqueued on qdisc queue, in
sock_queue_rcv_skb(), in __nf_queue().

Use skb_dst_force() in dev_requeue_skb().

Note: dst_use_noref() still dirties dst, we might transform it
later to do one dirtying per jiffies.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 107 insertions, 12 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c9525bce80f..7cdfb4d5284 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -264,7 +264,7 @@ typedef unsigned char *sk_buff_data_t;
  *	@transport_header: Transport layer header
  *	@network_header: Network layer header
  *	@mac_header: Link layer header
- *	@_skb_dst: destination entry
+ *	@_skb_refdst: destination entry (with norefcount bit)
  *	@sp: the security path, used for xfrm
  *	@cb: Control buffer. Free for use by every layer. Put private vars here
  *	@len: Length of actual data
@@ -328,7 +328,7 @@ struct sk_buff {
 	 */
 	char			cb[48] __aligned(8);
 
-	unsigned long		_skb_dst;
+	unsigned long		_skb_refdst;
 #ifdef CONFIG_XFRM
 	struct	sec_path	*sp;
 #endif
@@ -419,14 +419,64 @@ struct sk_buff {
 
 #include <asm/system.h>
 
+/*
+ * skb might have a dst pointer attached, refcounted or not.
+ * _skb_refdst low order bit is set if refcount was _not_ taken
+ */
+#define SKB_DST_NOREF	1UL
+#define SKB_DST_PTRMASK	~(SKB_DST_NOREF)
+
+/**
+ * skb_dst - returns skb dst_entry
+ * @skb: buffer
+ *
+ * Returns skb dst_entry, regardless of reference taken or not.
+ */
 static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
 {
-	return (struct dst_entry *)skb->_skb_dst;
+	/* If refdst was not refcounted, check we still are in a 
+	 * rcu_read_lock section
+	 */
+	WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
+		!rcu_read_lock_held() &&
+		!rcu_read_lock_bh_held());
+	return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
 }
 
+/**
+ * skb_dst_set - sets skb dst
+ * @skb: buffer
+ * @dst: dst entry
+ *
+ * Sets skb dst, assuming a reference was taken on dst and should
+ * be released by skb_dst_drop()
+ */
 static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
 {
-	skb->_skb_dst = (unsigned long)dst;
+	skb->_skb_refdst = (unsigned long)dst;
+}
+
+/**
+ * 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
+ */
+static inline void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
+{
+	WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
+	skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
+}
+
+/**
+ * skb_dst_is_noref - Test if skb dst isnt refcounted
+ * @skb: buffer
+ */
+static inline bool skb_dst_is_noref(const struct sk_buff *skb)
+{
+	return (skb->_skb_refdst & SKB_DST_NOREF) && skb_dst(skb);
 }
 
 static inline struct rtable *skb_rtable(const struct sk_buff *skb)
diff --git a/include/net/dst.h b/include/net/dst.h
index aac5a5fcfda..27207a13f2a 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -168,6 +168,12 @@ static inline void dst_use(struct dst_entry *dst, unsigned long time)
 	dst->lastuse = time;
 }
 
+static inline void dst_use_noref(struct dst_entry *dst, unsigned long time)
+{
+	dst->__use++;
+	dst->lastuse = time;
+}
+
 static inline
 struct dst_entry * dst_clone(struct dst_entry * dst)
 {
@@ -177,11 +183,47 @@ struct dst_entry * dst_clone(struct dst_entry * dst)
 }
 
 extern void dst_release(struct dst_entry *dst);
+
+static inline void refdst_drop(unsigned long refdst)
+{
+	if (!(refdst & SKB_DST_NOREF))
+		dst_release((struct dst_entry *)(refdst & SKB_DST_PTRMASK));
+}
+
+/**
+ * skb_dst_drop - drops skb dst
+ * @skb: buffer
+ *
+ * Drops dst reference count if a reference was taken.
+ */
 static inline void skb_dst_drop(struct sk_buff *skb)
 {
-	if (skb->_skb_dst)
-		dst_release(skb_dst(skb));
-	skb->_skb_dst = 0UL;
+	if (skb->_skb_refdst) {
+		refdst_drop(skb->_skb_refdst);
+		skb->_skb_refdst = 0UL;
+	}
+}
+
+static inline void skb_dst_copy(struct sk_buff *nskb, const struct sk_buff *oskb)
+{
+	nskb->_skb_refdst = oskb->_skb_refdst;
+	if (!(nskb->_skb_refdst & SKB_DST_NOREF))
+		dst_clone(skb_dst(nskb));
+}
+
+/**
+ * skb_dst_force - makes sure skb dst is refcounted
+ * @skb: buffer
+ *
+ * If dst is not yet refcounted, let's do it
+ */
+static inline void skb_dst_force(struct sk_buff *skb)
+{
+	if (skb_dst_is_noref(skb)) {
+		WARN_ON(!rcu_read_lock_held());
+		skb->_skb_refdst &= ~SKB_DST_NOREF;
+		dst_clone(skb_dst(skb));
+	}
 }
 
 /* Children define the path of the packet through the
diff --git a/include/net/sock.h b/include/net/sock.h
index aed16eb9db4..5697caf8cc7 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -600,12 +600,15 @@ static inline int sk_stream_memory_free(struct sock *sk)
 /* OOB backlog add */
 static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
-	if (!sk->sk_backlog.tail) {
-		sk->sk_backlog.head = sk->sk_backlog.tail = skb;
-	} else {
+	/* dont let skb dst not refcounted, we are going to leave rcu lock */
+	skb_dst_force(skb);
+
+	if (!sk->sk_backlog.tail)
+		sk->sk_backlog.head = skb;
+	else
 		sk->sk_backlog.tail->next = skb;
-		sk->sk_backlog.tail = skb;
-	}
+
+	sk->sk_backlog.tail = skb;
 	skb->next = NULL;
 }