From 3af6b2ad90138bec29f8949baa90b0d742c91eb4 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 23 Oct 2019 22:44:48 -0700 Subject: [PATCH] net: add skb_queue_empty_lockless() [ Upstream commit d7d16a89350ab263484c0aa2b523dd3a234e4a80 ] 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 Signed-off-by: David S. Miller Signed-off-by: Greg Kroah-Hartman --- include/linux/skbuff.h | 33 ++++++++++++++++++++++++--------- 1 file changed, 24 insertions(+), 9 deletions(-) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index fdb0cd0699b6..248806103fb7 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -1345,6 +1345,19 @@ static inline int skb_queue_empty(const struct sk_buff_head *list) return list->next == (const struct sk_buff *) list; } +/** + * 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 @@ -1709,9 +1722,11 @@ static inline void __skb_insert(struct sk_buff *newsk, 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++; } @@ -1722,11 +1737,11 @@ static inline void __skb_queue_splice(const struct sk_buff_head *list, 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); } /** @@ -1867,8 +1882,8 @@ static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list) 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); } /** -- 2.20.1