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

From: Joel Fernandes <[email-masked]>
Date: Tue, 4 Jun 2019 19:57:35 -0400

On Tue, Jun 04, 2019 at 04:01:00PM +0200, Rasmus Villemoes wrote:
> On 02/06/2019 00.27, Joel Fernandes (Google) wrote:
> > This patch adds support for checking RCU reader sections in list
> > traversal macros. Optionally, if the list macro is called under SRCU or
> > other lock/mutex protection, then appropriate lockdep expressions can be
> > passed to make the checks pass.
> > 
> > Existing list_for_each_entry_rcu() invocations don't need to pass the
> > optional fourth argument (cond) unless they are under some non-RCU
> > protection and needs to make lockdep check pass.
> > 
> > Signed-off-by: Joel Fernandes (Google) <[email-masked]>
> > ---
> >  include/linux/rculist.h  | 40 ++++++++++++++++++++++++++++++++++++----
> >  include/linux/rcupdate.h |  7 +++++++
> >  kernel/rcu/update.c      | 26 ++++++++++++++++++++++++++
> >  3 files changed, 69 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > index e91ec9ddcd30..b641fdd9f1a2 100644
> > --- a/include/linux/rculist.h
> > +++ b/include/linux/rculist.h
> > @@ -40,6 +40,25 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
> >   */
> >  #define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
> >  
> > +/*
> > + * Check during list traversal that we are within an RCU reader
> > + */
> > +#define __list_check_rcu()						\
> > +	RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),			\
> > +			 "RCU-list traversed in non-reader section!")
> > +
> > +static inline void __list_check_rcu_cond(int dummy, ...)
> > +{
> > +	va_list ap;
> > +	int cond;
> > +
> > +	va_start(ap, dummy);
> > +	cond = va_arg(ap, int);
> > +	va_end(ap);
> > +
> > +	RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),
> > +			 "RCU-list traversed in non-reader section!");
> > +}
> >  /*
> >   * Insert a new entry between two known consecutive entries.
> >   *
> > @@ -338,6 +357,9 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
> >  						  member) : NULL; \
> >  })
> >  
> > +#define SIXTH_ARG(a1, a2, a3, a4, a5, a6, ...) a6
> > +#define COUNT_VARGS(...) SIXTH_ARG(dummy, ## __VA_ARGS__, 4, 3, 2, 1, 0)
> > +>  /**
> >   * list_for_each_entry_rcu	-	iterate over rcu list of given type
> >   * @pos:	the type * to use as a loop cursor.
> > @@ -348,9 +370,14 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
> >   * 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_rcu((head)->next, typeof(*pos), member); \
> > -		&pos->member != (head); \
> > +#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))
> 
> Wouldn't something as simple as
> 
> #define __list_check_rcu(dummy, cond, ...) \
>        RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(), \
> 			 "RCU-list traversed in non-reader section!");
> 
> for ( ({ __list_check_rcu(junk, ##cond, 0); }), pos = ... )
> 
> work just as well (i.e., no need for two list_check_rcu and
> list_check_rcu_cond variants)? If there's an optional cond, we use that,
> if not, we pick the trailing 0, so !cond disappears and it reduces to
> your __list_check_rcu(). Moreover, this ensures the RCU_LOCKDEP_WARN
> expansion actually picks up the __LINE__ and __FILE__ where the for loop
> is used, and not the __FILE__ and __LINE__ of the static inline function
> from the header file. It also makes it a bit more type safe/type generic
> (if the cond expression happened to have type long or u64 something
> rather odd could happen with the inline vararg function).

This is much better. I will do it this way. Thank you!

 - Joel