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

From: Joel Fernandes <[email-masked]>
Date: Tue, 4 Jun 2019 13:48:02 -0400

On Tue, Jun 04, 2019 at 06:53:58AM -0400, Steven Rostedt wrote:
> On Mon, 3 Jun 2019 10:18:47 -0400
> Joel Fernandes <[email-masked]> 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),					\

Yes, already doing it this way as I replied to Peter here:
https://lore.kernel.org/patchwork/patch/1082846/#1278489

Thanks!

 - Joel