Re: [RFC 1/6] rcu: Add support for consolidated-RCU reader checking

[Steven Rostedt]

On Mon, 3 Jun 2019 10:18:47 -0400
Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> On Mon, Jun 03, 2019 at 10:01:28AM +0200, Peter Zijlstra wrote:
> > On Sat, Jun 01, 2019 at 06:27:33PM -0400, Joel Fernandes (Google) wrote:  
> > > +#define list_for_each_entry_rcu(pos, head, member, cond...)		\
> > > +	if (COUNT_VARGS(cond) != 0) {					\
> > > +		__list_check_rcu_cond(0, ## cond);			\
> > > +	} else {							\
> > > +		__list_check_rcu();					\
> > > +	}								\
> > > +	for (pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
> > > +		&pos->member != (head);					\
> > >  		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> > >  
> > >  /**
> > > @@ -621,7 +648,12 @@ static inline void hlist_add_behind_rcu(struct hlist_node *n,
> > >   * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> > >   * as long as the traversal is guarded by rcu_read_lock().
> > >   */
> > > +#define hlist_for_each_entry_rcu(pos, head, member, cond...)		\
> > > +	if (COUNT_VARGS(cond) != 0) {					\
> > > +		__list_check_rcu_cond(0, ## cond);			\
> > > +	} else {							\
> > > +		__list_check_rcu();					\
> > > +	}								\
> > >  	for (pos = hlist_entry_safe (rcu_dereference_raw(hlist_first_rcu(head)),\
> > >  			typeof(*(pos)), member);			\
> > >  		pos;							\  
> > 
> > 
> > This breaks code like:
> > 
> > 	if (...)
> > 		list_for_each_entry_rcu(...);
> > 
> > as they are no longer a single statement. You'll have to frob it into
> > the initializer part of the for statement.  
> 
> Thanks a lot for that. I fixed it as below (diff is on top of the patch):
> 
> If not for that '##' , I could have abstracted the whole if/else
> expression into its own macro and called it from list_for_each_entry_rcu() to
> keep it more clean.
> 
> ---8<-----------------------
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index b641fdd9f1a2..cc742d294bb0 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -371,12 +372,15 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
>  #define list_for_each_entry_rcu(pos, head, member, cond...)		\
> -	if (COUNT_VARGS(cond) != 0) {					\
> -		__list_check_rcu_cond(0, ## cond);			\
> -	} else {							\
> -		__list_check_rcu();					\
> -	}								\
> -	for (pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
> +	for (								\
> +	     ({								\
> +		if (COUNT_VARGS(cond) != 0) {				\
> +			__list_check_rcu_cond(0, ## cond);		\
> +		} else {						\
> +			__list_check_rcu_nocond();			\
> +		}							\
> +	      }),							\

For easier to read I would do something like this:

#define check_rcu_list(cond)						\
	({								\
		if (COUNT_VARGS(cond) != 0)				\
			__list_check_rcu_cond(0, ## cond);		\
		else							\
			__list_check_rcu_nocond();			\
	})

#define list_for_each_entry_rcu(pos, head, member, cond...)		\
	for (check_rcu_list(cond),					\


-- Steve

> +	     pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
>  		&pos->member != (head);					\
>  		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
>  
> @@ -649,12 +653,15 @@ static inline void hlist_add_behind_rcu(struct hlist_node *n,
>   * as long as the traversal is guarded by rcu_read_lock().
>   */
>  #define hlist_for_each_entry_rcu(pos, head, member, cond...)		\
> -	if (COUNT_VARGS(cond) != 0) {					\
> -		__list_check_rcu_cond(0, ## cond);			\
> -	} else {							\
> -		__list_check_rcu();					\
> -	}								\
> -	for (pos = hlist_entry_safe (rcu_dereference_raw(hlist_first_rcu(head)),\
> +	for (								\
> +	     ({								\
> +		if (COUNT_VARGS(cond) != 0) {				\
> +			__list_check_rcu_cond(0, ## cond);		\
> +		} else {						\
> +			__list_check_rcu_nocond();			\
> +		}							\
> +	     }),							\
> +	     pos = hlist_entry_safe (rcu_dereference_raw(hlist_first_rcu(head)),\
>  			typeof(*(pos)), member);			\
>  		pos;							\
>  		pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(\