Re: [LIST] Add missing rcu_dereference on first element

[Paul E. McKenney]

On Sat, Oct 15, 2005 at 10:26:49AM +1000, Herbert Xu wrote:
> Hi:
> 
> It seems that all the list_*_rcu primitives are missing a memory barrier
> on the very first dereference.  For example,
> 
> #define list_for_each_rcu(pos, head) \
> 	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
> 		pos = rcu_dereference(pos->next))
> 
> It will go something like:
> 
> 	pos = (head)->next
> 
> 	prefetch(pos->next)
> 
> 	pos != (head)
> 
> 	do stuff
> 
> We're missing a barrier here.
> 
> 	pos = rcu_dereference(pos->next)
> 
> 		fetch pos->next
> 
> 		barrier given by rcu_dereference(pos->next)
> 
> 		store pos
> 
> Without the missing barrier, the pos->next value may turn out to be
> stale.  In fact, if "do stuff" were also dereferencing pos and relying
> on list_for_each_rcu to provide the barrier then it may also break.
> 
> So here is a patch to make sure that we have a barrier for the first
> element in the list.

Good catch!!!  I wonder if Alpha SMP Linux machines have noticed...

Some comments interspersed.

> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> 
> Cheers,
> -- 
> Visit Openswan at http://www.openswan.org/
> Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

> diff --git a/include/linux/list.h b/include/linux/list.h
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -442,12 +442,15 @@ static inline void list_splice_init(stru
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
>  #define list_for_each_rcu(pos, head) \
> -	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
> -        	pos = rcu_dereference(pos->next))
> +	for (pos = (head)->next; \
> +		pos = rcu_dereference(pos), \
> +			prefetch(pos->next), pos != (head); \
> +        	pos = pos->next)

Why not something like the following?  Seems a bit simpler to me.

#define list_for_each_rcu(pos, head) \
	for (pos = rcu_dereference((head)->next); \
		prefetch(pos->next), pos != (head); \
        	pos = rcu_dereference(pos->next))

>  #define __list_for_each_rcu(pos, head) \
> -	for (pos = (head)->next; pos != (head); \
> -        	pos = rcu_dereference(pos->next))
> +	for (pos = (head)->next; \
> +		rcu_dereference(pos) != (head); \
> +        	pos = pos->next)
>  
>  /**
>   * list_for_each_safe_rcu	-	iterate over an rcu-protected list safe
> @@ -461,8 +464,9 @@ static inline void list_splice_init(stru
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
>  #define list_for_each_safe_rcu(pos, n, head) \
> -	for (pos = (head)->next, n = pos->next; pos != (head); \
> -		pos = rcu_dereference(n), n = pos->next)
> +	for (pos = (head)->next; \
> +		n = rcu_dereference(pos)->next, pos != (head); \
> +		pos = n)
>  
>  /**
>   * list_for_each_entry_rcu	-	iterate over rcu list of given type
> @@ -474,11 +478,11 @@ static inline void list_splice_init(stru
>   * the _rcu list-mutation primitives such as list_add_rcu()
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
> -#define list_for_each_entry_rcu(pos, head, member)			\
> -	for (pos = list_entry((head)->next, typeof(*pos), member);	\
> -	     prefetch(pos->member.next), &pos->member != (head); 	\
> -	     pos = rcu_dereference(list_entry(pos->member.next, 	\
> -					typeof(*pos), member)))
> +#define list_for_each_entry_rcu(pos, head, member) \
> +	for (pos = list_entry((head)->next, typeof(*pos), member); \
> +		pos = rcu_dereference(pos), \
> +			prefetch(pos->member.next), &pos->member != (head); \
> +		pos = list_entry(pos->member.next, typeof(*pos), member))
>  
>  
>  /**
> @@ -492,8 +496,10 @@ static inline void list_splice_init(stru
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
>  #define list_for_each_continue_rcu(pos, head) \
> -	for ((pos) = (pos)->next; prefetch((pos)->next), (pos) != (head); \
> -        	(pos) = rcu_dereference((pos)->next))
> +	for ((pos) = (pos)->next; \
> +		(pos) = rcu_dereference((pos)), \
> +			prefetch((pos)->next), (pos) != (head); \
> +        	(pos) = (pos)->next)

The above hurts my head -- childhood trauma due to having to use a
FORTRAN compiler that required "I=I" at odd intervals in order to
generate correct code...  How about the following?

#define list_for_each_continue_rcu(pos, head) \
	for ((pos) = (pos)->next; \
		prefetch(rcu_dereference(pos)->next), (pos) != (head); \
        	(pos) = (pos)->next)

>  /*
>   * Double linked lists with a single pointer list head.
> @@ -696,8 +702,9 @@ static inline void hlist_add_after_rcu(s
>  	     pos = n)
>  
>  #define hlist_for_each_rcu(pos, head) \
> -	for ((pos) = (head)->first; pos && ({ prefetch((pos)->next); 1; }); \
> -		(pos) = rcu_dereference((pos)->next))
> +	for ((pos) = (head)->first; \
> +		rcu_dereference((pos)) && ({ prefetch((pos)->next); 1; }); \
> +		(pos) = (pos)->next)
>  
>  /**
>   * hlist_for_each_entry	- iterate over list of given type
> @@ -762,9 +769,9 @@ static inline void hlist_add_after_rcu(s
>   */
>  #define hlist_for_each_entry_rcu(tpos, pos, head, member)		 \
>  	for (pos = (head)->first;					 \
> -	     pos && ({ prefetch(pos->next); 1;}) &&			 \
> +	     rcu_dereference(pos) && ({ prefetch(pos->next); 1;}) &&	 \
>  		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
> -	     pos = rcu_dereference(pos->next))
> +	     pos = pos->next)
>  
>  #else
>  #warning "don't include kernel headers in userspace"

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