[RFC 6/6] RCU: Track rcu_dereference() in RCU read-side critical section

[Boqun Feng]

The variables protected by an RCU read-side critical section are
sometimes hard to figure out, especially when the critical section is
long or has some function calls in it. However, figuring out which
variable a RCU read-side critical section protects could save
us a lot of time for code reviewing, bug fixing or performance tuning.

This patch therefore uses the LOCKED_ACCESS to collect the information
of relationship between rcu_dereference*() and rcu_read_lock*() by
doing:

Step 0: define a locked_access_class for RCU.

Step 1: set the content of rcu_*_lock_key and __srcu_key to the
address of the locked_access_class for RCU.

Step 2: add locked_access_point() in __rcu_dereference_check()

After that we can figure out not only in which RCU read-side critical
section but also after which rcu_read_lock*() called an
rcu_dereference*() is called.

This feature is controlled by a config option RCU_LOCKED_ACCESS.

Also clean up the initialization code of lockdep_maps for different
flavors of RCU a little bit.

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/rcupdate.h |  8 ++++++++
 include/linux/srcu.h     |  8 +++++++-
 kernel/locking/lockdep.c |  3 +++
 kernel/rcu/update.c      | 31 +++++++++++++++++--------------
 lib/Kconfig.debug        | 12 ++++++++++++
 5 files changed, 47 insertions(+), 15 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 14e6f47..4bab658 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -610,6 +610,13 @@ static inline void rcu_preempt_sleep_check(void)
 #define rcu_dereference_sparse(p, space)
 #endif /* #else #ifdef __CHECKER__ */
 
+#ifdef CONFIG_RCU_LOCKED_ACCESS
+extern struct locked_access_class rcu_laclass;
+#define rcu_dereference_access() \
+	locked_access_point(&rcu_laclass, LOCKED_ACCESS_TYPE_READ)
+#else /* #ifdef CONFIG_LOCKED_ACCESS */
+#define rcu_dereference_access()
+#endif /* #else #ifdef CONFIG_LOCKED_ACCESS */
 #define __rcu_access_pointer(p, space) \
 ({ \
 	typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
@@ -622,6 +629,7 @@ static inline void rcu_preempt_sleep_check(void)
 	typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
 	rcu_dereference_sparse(p, space); \
+	rcu_dereference_access(); \
 	((typeof(*p) __force __kernel *)(________p1)); \
 })
 #define __rcu_dereference_protected(p, c, space) \
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index f5f80c5..ff048a2 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -64,12 +64,18 @@ struct srcu_struct {
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
+# ifdef CONFIG_RCU_LOCKED_ACCESS
+# define RCU_KEY { .laclass = &rcu_laclass }
+# else
+# define RCU_KEY { 0 }
+# endif
+
 int __init_srcu_struct(struct srcu_struct *sp, const char *name,
 		       struct lock_class_key *key);
 
 #define init_srcu_struct(sp) \
 ({ \
-	static struct lock_class_key __srcu_key; \
+	static struct lock_class_key __srcu_key = RCU_KEY; \
 	\
 	__init_srcu_struct((sp), #sp, &__srcu_key); \
 })
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 996c2d5..36bd0cc 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4623,5 +4623,8 @@ void locked_access(struct locked_access_class *laclass,
 		correlate_locked_access(laclass, acqchain, loc, type);
 }
 EXPORT_SYMBOL(locked_access);
+#ifdef CONFIG_RCU_LOCKED_ACCESS
+DEFINE_LACLASS(rcu);
+#endif
 
 #endif /* CONFIG_LOCKED_ACCESS */
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 76b94e1..ba2ded6 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -235,20 +235,23 @@ EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
-static struct lock_class_key rcu_lock_key;
-struct lockdep_map rcu_lock_map =
-	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
-EXPORT_SYMBOL_GPL(rcu_lock_map);
-
-static struct lock_class_key rcu_bh_lock_key;
-struct lockdep_map rcu_bh_lock_map =
-	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
-EXPORT_SYMBOL_GPL(rcu_bh_lock_map);
-
-static struct lock_class_key rcu_sched_lock_key;
-struct lockdep_map rcu_sched_lock_map =
-	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
-EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
+
+# ifdef CONFIG_RCU_LOCKED_ACCESS
+# define RCU_KEY { .laclass = &rcu_laclass }
+# else
+# define RCU_KEY { 0 }
+# endif
+
+#define DEFINE_RCU_LOCKDEP_MAP(flavor)					\
+	static struct lock_class_key rcu##flavor##_lock_key = RCU_KEY;	\
+	struct lockdep_map rcu##flavor##_lock_map =			\
+		STATIC_LOCKDEP_MAP_INIT("rcu" #flavor "_read_lock",	\
+					&rcu##flavor##_lock_key);	\
+	EXPORT_SYMBOL_GPL(rcu##flavor##_lock_map)
+
+DEFINE_RCU_LOCKDEP_MAP();
+DEFINE_RCU_LOCKDEP_MAP(_bh);
+DEFINE_RCU_LOCKDEP_MAP(_sched);
 
 static struct lock_class_key rcu_callback_key;
 struct lockdep_map rcu_callback_map =
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 178f288..b6003d4 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1410,6 +1410,18 @@ config RCU_EQS_DEBUG
 	  Say N here if you need ultimate kernel/user switch latencies
 	  Say Y if you are unsure
 
+config RCU_LOCKED_ACCESS
+	bool "Track data access in RCU read-side critical sections"
+	depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT
+	select LOCKED_ACCESS
+	default n
+	help
+	  Track data acces in RCU read-side critical sections,
+	  by doing so, one can examine which rcu_dereference() and its
+	  friends are called in which RCU read-side critical sections,
+	  and even more detailed, after which rcu_read_lock() and
+	  its friends are called.
+
 endmenu # "RCU Debugging"
 
 config DEBUG_BLOCK_EXT_DEVT
-- 
2.7.0