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

On Tue, Jun 04, 2019 at 04:01:00PM +0200, Rasmus Villemoes wrote:

On 02/06/2019 00.27, Joel Fernandes (Google) wrote:

quoted

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) <redacted>
---
 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