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 df0b7e620659..5dbedcc571fd 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -960,24 +960,20 @@ 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) @@ -1066,6 +1062,18 @@ 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 @@ -1074,23 +1082,22 @@ 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: -- 2.14.1