The following commit has been merged into the locking/core branch of tip: Commit-ID: d563bc6ead9e79be37067d58509a605b67378184 Gitweb: https://git.kernel.org/tip/d563bc6ead9e79be37067d58509a605b67378184 Author: Boqun Feng <boqun.feng@xxxxxxxxx> AuthorDate: Fri, 07 Aug 2020 15:42:23 +08:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Wed, 26 Aug 2020 12:42:03 +02:00 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@xxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20200807074238.1632519-5-boqun.feng@xxxxxxxxx --- kernel/locking/lockdep.c | 61 ++++++++++++++++++++++----------------- 1 file changed, 35 insertions(+), 26 deletions(-) diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 462c68c..150686a 100644 --- 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:
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