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SubjectRe: [RFC 1/6] rcu: Add support for consolidated-RCU reader checking
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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@joelfernandes.org>
> ---
> 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

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