[PATCH 04/17] locking/lockdep: Update direct dependency's read-write type if it exists

[Yuyang Du <duyuyang@gmail.com>]

When adding a dependency, if the dependency exists - be it direct or
indirect - the dependency's read-write type may be updated.

We can keep all different-typed dependencies, but for simplicity we just
"upgrade" lock types if possible, making them more exlusive (in the order:
recursive read -> read -> write.

For indirect dependency, we do the redundancy check only when it has no
read locks (i.e., write-lock ended dependency); this makes the redundancy
check less complicated, which matters only when CONFIG_LOCKDEP_SMALL.

Signed-off-by: Yuyang Du <duyuyang@gmail.com>
---
 kernel/locking/lockdep.c | 61 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 57 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 0617375..27ca55f 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1734,6 +1734,16 @@ static inline int get_lock_type2(struct lock_list *lock)
 	return lock->lock_type[1];
 }
 
+static inline void set_lock_type1(struct lock_list *lock, int read)
+{
+	lock->lock_type[0] = (u16)read;
+}
+
+static inline void set_lock_type2(struct lock_list *lock, int read)
+{
+	lock->lock_type[1] = (u16)read;
+}
+
 /*
  * Check that the dependency graph starting at <src> can lead to
  * <target> or not. Print an error and return 0 if it does.
@@ -1814,6 +1824,32 @@ static inline int get_lock_type2(struct lock_list *lock)
 	ret = check_path(hlock_class(target), &src_entry, &target_entry);
 
 	if (!ret) {
+		struct lock_list *parent;
+
+		/*
+		 * Do this indirect dependency has the same type as the
+		 * direct dependency?
+		 *
+		 * Actually, we keep the more exclusive lock type
+		 * between the indirect and direct dependencies by
+		 * "upgrading" the indirect dependency's lock type if
+		 * needed, and then consider this redundant.
+		 */
+
+		/* Target end lock type: */
+		if (target->read < get_lock_type2(target_entry))
+			set_lock_type2(target_entry, target->read)
+
+		/* Source end lock type: */
+		parent = find_next_dep_in_path(&src_entry, target_entry);
+		if (!parent) {
+			DEBUG_LOCKS_WARN_ON(1);
+			return 0;
+		}
+
+		if (src->read < get_lock_type1(parent))
+			set_lock_type1(parent, src->read)
+
 		debug_atomic_inc(nr_redundant);
 		ret = 2;
 	} else if (ret < 0)
@@ -2479,6 +2515,17 @@ static inline void inc_chains(void)
 	 */
 	list_for_each_entry(entry, &hlock_class(prev)->locks_after, entry) {
 		if (entry->class == hlock_class(next)) {
+			/*
+			 * For direct dependency, smaller type value
+			 * generally means more exlusive; we keep the
+			 * more exlusive ones, in other words, we
+			 * "upgrade" the dependency if we can.
+			 */
+			if (prev->read < get_lock_type1(entry))
+				set_lock_type1(entry, prev->read);
+			if (next->read < get_lock_type2(entry))
+				set_lock_type2(entry, next->read);
+
 			if (distance == 1)
 				entry->distance = 1;
 			return 1;
@@ -2487,11 +2534,17 @@ static inline void inc_chains(void)
 
 #ifdef CONFIG_LOCKDEP_SMALL
 	/*
-	 * Is the <prev> -> <next> link redundant?
+	 * Only when this dependency has no read lock/locks (i.e.,
+	 * write-write dependency), we do redundancy check.
 	 */
-	ret = check_redundant(prev, next);
-	if (ret != 1)
-		return ret;
+	if (!get_dep_type(prev, next)) {
+		/*
+		 * Is the <prev> -> <next> link redundant?
+		 */
+		ret = check_redundant(prev, next);
+		if (ret != 1)
+			return ret;
+	}
 #endif
 
 	if (!trace->nr_entries && !save_trace(trace))
-- 
1.8.3.1