[RFC tip/locking v2 04/13] lockdep: Introduce lock_list::dep

[Boqun Feng]

To add recursive read locks into the dependency graph, we need to store
the types of dependencies for the BFS later. There are four kinds of
dependencies:

*	Non-recursive -> Non-recursive dependencies(NN)
	e.g. write_lock(prev) -> write_lock(next)

*	Recursive -> Non-recursive dependencies(RN)
	e.g. read_lock(prev) -> write_lock(next);

*	Non-recursive -> recursive dependencies(NR)
	e.g. write_lock(prev) -> read_lock(next);

*	Recursive -> recursive dependencies(RR)
	e.g. read_lock(prev) -> read_lock(next);

Given a pair of two locks, four kinds of dependencies could all exist
between them, so we use 4 bit for the presence of each kind(stored in
lock_list::dep).

Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 include/linux/lockdep.h  |  2 ++
 kernel/locking/lockdep.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index bfa8e0b0d6f1..35b3fc0d6793 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -192,6 +192,8 @@ struct lock_list {
 	struct lock_class		*class;
 	struct stack_trace		trace;
 	int				distance;
+	/* bitmap of different dependencies from head to this */
+	u16				dep;
 
 	/*
 	 * The parent field is used to implement breadth-first search, and the
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 86ef7ea9f79f..1220dab7a506 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -884,6 +884,7 @@ static int add_lock_to_list(struct lock_class *this, struct list_head *head,
 		return 0;
 
 	entry->class = this;
+	entry->dep = dep;
 	entry->distance = distance;
 	entry->trace = *trace;
 	/*
@@ -1024,6 +1025,33 @@ static inline bool bfs_error(enum bfs_result res)
 	return res < 0;
 }
 
+#define DEP_NN_BIT 0
+#define DEP_RN_BIT 1
+#define DEP_NR_BIT 2
+#define DEP_RR_BIT 3
+
+#define DEP_NN_MASK (1U << (DEP_NN_BIT))
+#define DEP_RN_MASK (1U << (DEP_RN_BIT))
+#define DEP_NR_MASK (1U << (DEP_NR_BIT))
+#define DEP_RR_MASK (1U << (DEP_RR_BIT))
+
+static inline unsigned int __calc_dep_bit(int prev, int next)
+{
+	if (prev == 2 && next != 2)
+		return DEP_RN_BIT;
+	if (prev != 2 && next == 2)
+		return DEP_NR_BIT;
+	if (prev == 2 && next == 2)
+		return DEP_RR_BIT;
+	else
+		return DEP_NN_BIT;
+}
+
+static inline unsigned int calc_dep(int prev, int next)
+{
+	return 1U << __calc_dep_bit(prev, next);
+}
+
 static enum bfs_result __bfs(struct lock_list *source_entry,
 			     void *data,
 			     int (*match)(struct lock_list *entry, void *data),
@@ -1951,6 +1979,16 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 		if (entry->class == hlock_class(next)) {
 			if (distance == 1)
 				entry->distance = 1;
+			entry->dep |= calc_dep(prev->read, next->read);
+		}
+	}
+
+	/* Also, update the reverse dependency in @next's ->locks_before list */
+	list_for_each_entry(entry, &hlock_class(next)->locks_before, entry) {
+		if (entry->class == hlock_class(prev)) {
+			if (distance == 1)
+				entry->distance = 1;
+			entry->dep |= calc_dep(next->read, prev->read);
 			return 1;
 		}
 	}
@@ -1978,14 +2016,18 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 	 */
 	ret = add_lock_to_list(hlock_class(next),
 			       &hlock_class(prev)->locks_after,
-			       next->acquire_ip, distance, trace);
+			       next->acquire_ip, distance,
+			       calc_dep(prev->read, next->read),
+			       trace);
 
 	if (!ret)
 		return 0;
 
 	ret = add_lock_to_list(hlock_class(prev),
 			       &hlock_class(next)->locks_before,
-			       next->acquire_ip, distance, trace);
+			       next->acquire_ip, distance,
+			       calc_dep(next->read, prev->read),
+			       trace);
 	if (!ret)
 		return 0;
 
-- 
2.14.1