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SubjectRe: [RFC 1/6] rcu: Add support for consolidated-RCU reader checking
On Mon, 3 Jun 2019 10:18:47 -0400
Joel Fernandes <joel@joelfernandes.org> 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(\

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