locking/lockdep: Introduce lock_sync()

Currently, functions like synchronize_srcu() do not have lockdep
annotations resembling those of other write-side locking primitives.
Such annotations might look as follows:

	lock_acquire();
	lock_release();

Such annotations would tell lockdep that synchronize_srcu() acts like
an empty critical section that waits for other (read-side) critical
sections to finish.  This would definitely catch some deadlock, but
as pointed out by Paul Mckenney [1], this could also introduce false
positives because of irq-safe/unsafe detection.  Of course, there are
tricks could help with this:

	might_sleep(); // Existing statement in __synchronize_srcu().
	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
		local_irq_disable();
		lock_acquire();
		lock_release();
		local_irq_enable();
	}

But it would be better for lockdep to provide a separate annonation for
functions like synchronize_srcu(), so that people won't need to repeat
the ugly tricks above.

Therefore introduce lock_sync(), which is simply an lock+unlock
pair with no irq safe/unsafe deadlock check.  This works because the
to-be-annontated functions do not create real critical sections, and
there is therefore 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>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ boqun: Fix typos reported by Davidlohr Bueso and Paul E. Mckenney ]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 1023f34..14d9dbe 100644 (file)
--- 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
@@ -554,6 +558,7 @@ 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 50d4863..3ee3b27 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5693,6 +5693,40 @@ 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 acquire+release annotation 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;