[PATCH 1/3] locking/lockdep: Introduce lock_sync()

[Boqun Feng <boqun.feng@gmail.com>]

Currently, in order to annonate functions like synchronize_srcu() for
lockdep, a trick as follow can be used:

	lock_acquire();
	lock_release();

, which indicates synchronize_srcu() acts like an empty critical section
that waits for other (read-side) critical sections to finish. This
surely can catch some deadlock, but as discussion brought up by Paul
Mckenney [1], this could introduce false positives because of
irq-safe/unsafe detection. Extra tricks might help this:

	local_irq_disable(..);
	lock_acquire();
	lock_release();
	local_irq_enable(...);

But it's better that lockdep could provide an annonation for
synchronize_srcu() like functions, so that people won't need to repeat
the ugly tricks above. Therefore introduce lock_sync(). It's simply an
lock+unlock pair with no irq safe/unsafe deadlock check, since the
to-be-annontated functions don't create real critical sections therefore
there is no way that irq can create extra dependencies.

[1]: https://lore.kernel.org/lkml/20180412021233.ewncg5jjuzjw3x62@tardis/

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 include/linux/lockdep.h  |  5 +++++
 kernel/locking/lockdep.c | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 1f1099dac3f0..ba09df6a0872 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -268,6 +268,10 @@ extern void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
 
 extern void lock_release(struct lockdep_map *lock, unsigned long ip);
 
+extern void lock_sync(struct lockdep_map *lock, unsigned int subclass,
+		      int read, int check, struct lockdep_map *nest_lock,
+		      unsigned long ip);
+
 /* lock_is_held_type() returns */
 #define LOCK_STATE_UNKNOWN	-1
 #define LOCK_STATE_NOT_HELD	0
@@ -555,6 +559,7 @@ do {									\
 #define lock_map_acquire_read(l)		lock_acquire_shared_recursive(l, 0, 0, NULL, _THIS_IP_)
 #define lock_map_acquire_tryread(l)		lock_acquire_shared_recursive(l, 0, 1, NULL, _THIS_IP_)
 #define lock_map_release(l)			lock_release(l, _THIS_IP_)
+#define lock_map_sync(l)			lock_sync(l, 0, 0, 1, NULL, _THIS_IP_)
 
 #ifdef CONFIG_PROVE_LOCKING
 # define might_lock(lock)						\
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index e3375bc40dad..cffa026a765f 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5692,6 +5692,40 @@ void lock_release(struct lockdep_map *lock, unsigned long ip)
 }
 EXPORT_SYMBOL_GPL(lock_release);
 
+/*
+ * lock_sync() - A special annotation for synchronize_{s,}rcu()-like API.
+ *
+ * No actual critical section is created by the APIs annotated with this: these
+ * APIs are used to wait for one or multiple critical sections (on other CPUs
+ * or threads), and it means that calling these APIs inside these critical
+ * sections is potential deadlock.
+ *
+ * This annotation acts as an acqurie+release anontation pair with hardirqoff
+ * being 1. Since there's no critical section, no interrupt can create extra
+ * dependencies "inside" the annotation, hardirqoff == 1 allows us to avoid
+ * false positives.
+ */
+void lock_sync(struct lockdep_map *lock, unsigned subclass, int read,
+	       int check, struct lockdep_map *nest_lock, unsigned long ip)
+{
+	unsigned long flags;
+
+	if (unlikely(!lockdep_enabled()))
+		return;
+
+	raw_local_irq_save(flags);
+	check_flags(flags);
+
+	lockdep_recursion_inc();
+	__lock_acquire(lock, subclass, 0, read, check, 1, nest_lock, ip, 0, 0);
+
+	if (__lock_release(lock, ip))
+		check_chain_key(current);
+	lockdep_recursion_finish();
+	raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_sync);
+
 noinstr int lock_is_held_type(const struct lockdep_map *lock, int read)
 {
 	unsigned long flags;
-- 
2.38.1