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

From: Joel Fernandes <[email-masked]>
Date: Mon, 3 Jun 2019 10:18:47 -0400

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();			\
+		}							\
+	      }),							\
+	     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(\
-- 
2.22.0.rc1.311.g5d7573a151-goog