lockdep: Make __bfs() visit every dependency until a match

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 dependencies 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, as a result, 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.

Note: the checks in mark_lock_accessed() and lock_accessed() are
removed, because after this modification, we may call these two
functions on @source_entry of __bfs(), which may not be the entry in
"list_entries"

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200807074238.1632519-5-boqun.feng@gmail.com

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 462c68cfb378a7c1a2bacc23663e9325db893ec6..150686a71be04b0d9a29dada714c4f31af89131a 100644 (file)
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1421,23 +1421,19 @@ 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_accessed(struct lock_list *lock)
 {
-       unsigned long nr;
+       lock->class->dep_gen_id = lockdep_dependency_gen_id;
+}
 
-       nr = lock - list_entries;
-       WARN_ON(nr >= ARRAY_SIZE(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)
 {
-       unsigned long nr;
-
-       nr = lock - list_entries;
-       WARN_ON(nr >= ARRAY_SIZE(list_entries)); /* Out-of-bounds, input fail */
        return lock->class->dep_gen_id == lockdep_dependency_gen_id;
 }
 
@@ -1540,26 +1536,39 @@ 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_accessed(lock))
+                       continue;
+               else
+                       mark_lock_accessed(lock);
+
                head = get_dep_list(lock, offset);
 
+               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;
-                               }
-
-                               if (__cq_enqueue(cq, 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;
+                       unsigned int cq_depth;
+
+                       visit_lock_entry(entry, lock);
+                       if (match(entry, data)) {
+                               *target_entry = entry;
+                               ret = BFS_RMATCH;
+                               goto exit;
+                       }
+
+                       if (__cq_enqueue(cq, 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: