From: Eric Dumazet <edumazet@google.com>
Date: Wed, 23 Oct 2019 22:44:48 -0700
Subject: net: add skb_queue_empty_lockless()
Git-commit: d7d16a89350ab263484c0aa2b523dd3a234e4a80
Patch-mainline: 5.4-rc6
References: networking-stable-19_11_05

Some paths call skb_queue_empty() without holding
the queue lock. We must use a barrier in order
to not let the compiler do strange things, and avoid
KCSAN splats.

Adding a barrier in skb_queue_empty() might be overkill,
I prefer adding a new helper to clearly identify
points where the callers might be lockless. This might
help us finding real bugs.

The corresponding WRITE_ONCE() should add zero cost
for current compilers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 include/linux/skbuff.h |   33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1211,6 +1211,19 @@ static inline int skb_queue_empty(const
 }
 
 /**
+ *	skb_queue_empty_lockless - check if a queue is empty
+ *	@list: queue head
+ *
+ *	Returns true if the queue is empty, false otherwise.
+ *	This variant can be used in lockless contexts.
+ */
+static inline bool skb_queue_empty_lockless(const struct sk_buff_head *list)
+{
+	return READ_ONCE(list->next) == (const struct sk_buff *) list;
+}
+
+
+/**
  *	skb_queue_is_last - check if skb is the last entry in the queue
  *	@list: queue head
  *	@skb: buffer
@@ -1574,9 +1587,11 @@ static inline void __skb_insert(struct s
 				struct sk_buff *prev, struct sk_buff *next,
 				struct sk_buff_head *list)
 {
-	newsk->next = next;
-	newsk->prev = prev;
-	next->prev  = prev->next = newsk;
+	/* see skb_queue_empty_lockless() for the opposite READ_ONCE() */
+	WRITE_ONCE(newsk->next, next);
+	WRITE_ONCE(newsk->prev, prev);
+	WRITE_ONCE(next->prev, newsk);
+	WRITE_ONCE(prev->next, newsk);
 	list->qlen++;
 }
 
@@ -1587,11 +1602,11 @@ static inline void __skb_queue_splice(co
 	struct sk_buff *first = list->next;
 	struct sk_buff *last = list->prev;
 
-	first->prev = prev;
-	prev->next = first;
+	WRITE_ONCE(first->prev, prev);
+	WRITE_ONCE(prev->next, first);
 
-	last->next = next;
-	next->prev = last;
+	WRITE_ONCE(last->next, next);
+	WRITE_ONCE(next->prev, last);
 }
 
 /**
@@ -1732,8 +1747,8 @@ static inline void __skb_unlink(struct s
 	next	   = skb->next;
 	prev	   = skb->prev;
 	skb->next  = skb->prev = NULL;
-	next->prev = prev;
-	prev->next = next;
+	WRITE_ONCE(next->prev, prev);
+	WRITE_ONCE(prev->next, next);
 }
 
 /**