[PATCH rcu v2 2/7] rcu: Annotate SRCU's update-side lockdep dependencies

[Boqun Feng]

Although all flavors of RCU readers are annotated correctly with
lockdep as recursive read locks, they do not set the lock_acquire
'check' parameter.  This means that RCU read locks are not added to
the lockdep dependency graph, which in turn means that lockdep cannot
detect RCU-based deadlocks.  This is not a problem for RCU flavors having
atomic read-side critical sections because context-based annotations can
catch these deadlocks, see for example the RCU_LOCKDEP_WARN() statement
in synchronize_rcu().  But context-based annotations are not helpful
for sleepable RCU, especially given that it is perfectly legal to do
synchronize_srcu(&srcu1) within an srcu_read_lock(&srcu2).

However, we can detect SRCU-based by: (1) Making srcu_read_lock() a
'check'ed recursive read lock and (2) Making synchronize_srcu() a empty
write lock critical section.  Even better, with the newly introduced
lock_sync(), we can avoid false positives about irq-unsafe/safe.
This commit therefore makes it so.

Note that NMI-safe SRCU read side critical sections are currently not
annotated, but might be annotated in the future.

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
[ boqun: Add comments for annotation per Waiman's suggestion ]
[ boqun: Fix comment warning reported by Stephen Rothwell ]
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/srcu.h  | 34 ++++++++++++++++++++++++++++++++--
 kernel/rcu/srcutiny.c |  2 ++
 kernel/rcu/srcutree.c |  2 ++
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 74796cd7e7a9..41c4b26fb1c1 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -102,6 +102,32 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
 	return lock_is_held(&ssp->dep_map);
 }
 
+/*
+ * Annotations provide deadlock detection for SRCU.
+ *
+ * Similar to other lockdep annotations, except there is an additional
+ * srcu_lock_sync(), which is basically an empty *write*-side critical section,
+ * see lock_sync() for more information.
+ */
+
+/* Annotates a srcu_read_lock() */
+static inline void srcu_lock_acquire(struct lockdep_map *map)
+{
+	lock_map_acquire_read(map);
+}
+
+/* Annotates a srcu_read_lock() */
+static inline void srcu_lock_release(struct lockdep_map *map)
+{
+	lock_map_release(map);
+}
+
+/* Annotates a synchronize_srcu() */
+static inline void srcu_lock_sync(struct lockdep_map *map)
+{
+	lock_map_sync(map);
+}
+
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
@@ -109,6 +135,10 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
 	return 1;
 }
 
+#define srcu_lock_acquire(m) do { } while (0)
+#define srcu_lock_release(m) do { } while (0)
+#define srcu_lock_sync(m) do { } while (0)
+
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 #define SRCU_NMI_UNKNOWN	0x0
@@ -182,7 +212,7 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
 
 	srcu_check_nmi_safety(ssp, false);
 	retval = __srcu_read_lock(ssp);
-	rcu_lock_acquire(&(ssp)->dep_map);
+	srcu_lock_acquire(&(ssp)->dep_map);
 	return retval;
 }
 
@@ -254,7 +284,7 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
 {
 	WARN_ON_ONCE(idx & ~0x1);
 	srcu_check_nmi_safety(ssp, false);
-	rcu_lock_release(&(ssp)->dep_map);
+	srcu_lock_release(&(ssp)->dep_map);
 	__srcu_read_unlock(ssp, idx);
 }
 
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index b12fb0cec44d..336af24e0fe3 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -197,6 +197,8 @@ void synchronize_srcu(struct srcu_struct *ssp)
 {
 	struct rcu_synchronize rs;
 
+	srcu_lock_sync(&ssp->dep_map);
+
 	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
 			lock_is_held(&rcu_bh_lock_map) ||
 			lock_is_held(&rcu_lock_map) ||
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index ab4ee58af84b..c541b82646b6 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1307,6 +1307,8 @@ static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
 {
 	struct rcu_synchronize rcu;
 
+	srcu_lock_sync(&ssp->dep_map);
+
 	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
 			 lock_is_held(&rcu_bh_lock_map) ||
 			 lock_is_held(&rcu_lock_map) ||
-- 
2.38.1