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Commit d7d16a89 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

net: add skb_queue_empty_lockless() 



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: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent fc11078d

include/linux/skbuff.h

+24 −9
Original line number Diff line number Diff line
@@ -1495,6 +1495,19 @@ 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
@@ -1848,9 +1861,11 @@ 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++;

}
@@ -1861,11 +1876,11 @@ 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);

}



/**
@@ -2006,8 +2021,8 @@ 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);

}



/**