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

[Rasmus Villemoes]

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) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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).

Rasmus