Currently, __bfs() will do a breadth-first search in the dependency
graph and visit each lock class in the graph exactly once, so for
example, in the following graph:

        A ---------> B
        |            ^
        |            |
        +----------> C

a __bfs() call starts at A, will visit B through dependency A -> B and
visit C through dependency A -> C and that's it, IOW, __bfs() will not
visit dependency C -> B.

This is OK for now, as we only have strong dependencies in the
dependency graph, so whenever there is a traverse path from A to B in
__bfs(), it means A has strong dependency to B (IOW, B depends on A
strongly). So no need to visit all dependencies in the graph.

However, as we are going to add recursive-read lock into the dependency
graph, afterwards, not all the paths mean strong dependencies, in the
same example above, dependency A -> B may be a weak dependency and
traverse A -> C -> B may be a strong dependency path. And with the old
way of __bfs() (i.e. visiting every lock class exactly once), we will
miss the strong dependency path, which will result into failing to find
a deadlock. To cure this for the future, we need to find a way for
__bfs() to visit each dependency, rather than each class, exactly once
in the search until we find a match.

The solution is simple:

We used to mark lock_class::lockdep_dependency_gen_id to indicate a
class has been visited in __bfs(), now we change the semantics a little
bit: we now mark lock_class::lockdep_dependency_gen_id to indicate _all
the dependencies_ in its lock_{after,before} have been visited in the
__bfs() (note we only take one direction in a __bfs() search). In this
way, every dependency is guaranteed to be visited until we find a match.

Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
---
 kernel/locking/lockdep.c | 61 +++++++++++++++++++++++++++---------------------
 1 file changed, 34 insertions(+), 27 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 2dbaff381778..f39a071ef0a8 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -948,24 +948,20 @@ static inline unsigned int  __cq_get_elem_count(struct 
circular_queue *cq)
        return (cq->rear - cq->front) & CQ_MASK;
 }
 
-static inline void mark_lock_accessed(struct lock_list *lock,
-                                       struct lock_list *parent)
+static inline void mark_lock_list_accessed(struct lock_class *class)
 {
-       unsigned long nr;
+       class->dep_gen_id = lockdep_dependency_gen_id;
+}
 
-       nr = lock - list_entries;
-       WARN_ON(nr >= nr_list_entries); /* Out-of-bounds, input fail */
+static inline void visit_lock_entry(struct lock_list *lock,
+                                   struct lock_list *parent)
+{
        lock->parent = parent;
-       lock->class->dep_gen_id = lockdep_dependency_gen_id;
 }
 
-static inline unsigned long lock_accessed(struct lock_list *lock)
+static inline unsigned long lock_list_accessed(struct lock_class *class)
 {
-       unsigned long nr;
-
-       nr = lock - list_entries;
-       WARN_ON(nr >= nr_list_entries); /* Out-of-bounds, input fail */
-       return lock->class->dep_gen_id == lockdep_dependency_gen_id;
+       return class->dep_gen_id == lockdep_dependency_gen_id;
 }
 
 static inline struct lock_list *get_lock_parent(struct lock_list *child)
@@ -1054,6 +1050,18 @@ static enum bfs_result __bfs(struct lock_list 
*source_entry,
                        goto exit;
                }
 
+               /*
+                * If we have visited all the dependencies from this @lock to
+                * others (iow, if we have visited all lock_list entries in
+                * @lock->class->locks_{after,before}) we skip, otherwise go
+                * and visit all the dependencies in the list and mark this
+                * list accessed.
+                */
+               if (lock_list_accessed(lock->class))
+                       continue;
+               else
+                       mark_lock_list_accessed(lock->class);
+
                if (forward)
                        head = &lock->class->locks_after;
                else
@@ -1062,23 +1070,22 @@ static enum bfs_result __bfs(struct lock_list 
*source_entry,
                DEBUG_LOCKS_WARN_ON(!irqs_disabled());
 
                list_for_each_entry_rcu(entry, head, entry) {
-                       if (!lock_accessed(entry)) {
-                               unsigned int cq_depth;
-                               mark_lock_accessed(entry, lock);
-                               if (match(entry, data)) {
-                                       *target_entry = entry;
-                                       ret = BFS_RMATCH;
-                                       goto exit;
-                               }
+                       unsigned int cq_depth;
 
-                               if (__cq_enqueue(cq, (unsigned long)entry)) {
-                                       ret = BFS_EQUEUEFULL;
-                                       goto exit;
-                               }
-                               cq_depth = __cq_get_elem_count(cq);
-                               if (max_bfs_queue_depth < cq_depth)
-                                       max_bfs_queue_depth = cq_depth;
+                       visit_lock_entry(entry, lock);
+                       if (match(entry, data)) {
+                               *target_entry = entry;
+                               ret = BFS_RMATCH;
+                               goto exit;
+                       }
+
+                       if (__cq_enqueue(cq, (unsigned long)entry)) {
+                               ret = BFS_EQUEUEFULL;
+                               goto exit;
                        }
+                       cq_depth = __cq_get_elem_count(cq);
+                       if (max_bfs_queue_depth < cq_depth)
+                               max_bfs_queue_depth = cq_depth;
                }
        }
 exit:
-- 
2.16.2

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