lockdep: Optimize the memory usage of circular queue

Qian Cai reported a BFS_EQUEUEFULL warning [1] after read recursive
deadlock detection merged into tip tree recently. Unlike the previous
lockep graph searching, which iterate every lock class (every node in
the graph) exactly once, the graph searching for read recurisve deadlock
detection needs to iterate every lock dependency (every edge in the
graph) once, as a result, the maximum memory cost of the circular queue
changes from O(V), where V is the number of lock classes (nodes or
vertices) in the graph, to O(E), where E is the number of lock
dependencies (edges), because every lock class or dependency gets
enqueued once in the BFS. Therefore we hit the BFS_EQUEUEFULL case.

However, actually we don't need to enqueue all dependencies for the BFS,
because every time we enqueue a dependency, we almostly enqueue all
other dependencies in the same dependency list ("almostly" is because
we currently check before enqueue, so if a dependency doesn't pass the
check stage we won't enqueue it, however, we can always do in reverse
ordering), based on this, we can only enqueue the first dependency from
a dependency list and every time we want to fetch a new dependency to
work, we can either:

  1)	fetch the dependency next to the current dependency in the
	dependency list
or

  2)	if the dependency in 1) doesn't exist, fetch the dependency from
	the queue.

With this approach, the "max bfs queue depth" for a x86_64_defconfig +
lockdep and selftest config kernel can get descreased from:

        max bfs queue depth:                   201

to (after apply this patch)

        max bfs queue depth:                   61

While I'm at it, clean up the code logic a little (e.g. directly return
other than set a "ret" value and goto the "exit" label).

[1]: https://lore.kernel.org/lkml/17343f6f7f2438fc376125384133c5ba70c2a681.camel@redhat.com/Reported-by: Qian Cai <cai@redhat.com>
Reported-by: syzbot+62ebe501c1ce9a91f68c@syzkaller.appspotmail.com
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200917080210.108095-1-boqun.feng@gmail.com

kernel/locking/lockdep.c

@@ -1606,6 +1606,15 @@ static inline void bfs_init_rootb(struct lock_list *lock,
 	lock->only_xr = (hlock->read != 0);
 }
 
+static inline struct lock_list *__bfs_next(struct lock_list *lock, int offset)
+{
+	if (!lock || !lock->parent)
+		return NULL;
+
+	return list_next_or_null_rcu(get_dep_list(lock->parent, offset),
+				     &lock->entry, struct lock_list, entry);
+}
+
 /*
  * Breadth-First Search to find a strong path in the dependency graph.
  *
@@ -1639,36 +1648,25 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
 			     struct lock_list **target_entry,
 			     int offset)
 {
+	struct circular_queue *cq = &lock_cq;
+	struct lock_list *lock = NULL;
 	struct lock_list *entry;
-	struct lock_list *lock;
 	struct list_head *head;
-	struct circular_queue *cq = &lock_cq;
-	enum bfs_result ret = BFS_RNOMATCH;
+	unsigned int cq_depth;
+	bool first;
 
 	lockdep_assert_locked();
 
-	if (match(source_entry, data)) {
-		*target_entry = source_entry;
-		ret = BFS_RMATCH;
-		goto exit;
-	}
-
-	head = get_dep_list(source_entry, offset);
-	if (list_empty(head))
-		goto exit;
-
 	__cq_init(cq);
 	__cq_enqueue(cq, source_entry);
 
-	while ((lock = __cq_dequeue(cq))) {
-		bool prev_only_xr;
-
-		if (!lock->class) {
-			ret = BFS_EINVALIDNODE;
-			goto exit;
-		}
+	while ((lock = __bfs_next(lock, offset)) || (lock = __cq_dequeue(cq))) {
+		if (!lock->class)
+			return BFS_EINVALIDNODE;
 
 		/*
+		 * Step 1: check whether we already finish on this one.
+		 *
 		 * 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
@@ -1680,13 +1678,13 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
 		else
 			mark_lock_accessed(lock);
 
-		head = get_dep_list(lock, offset);
-
-		prev_only_xr = lock->only_xr;
-
-		list_for_each_entry_rcu(entry, head, entry) {
-			unsigned int cq_depth;
-			u8 dep = entry->dep;
+		/*
+		 * Step 2: check whether prev dependency and this form a strong
+		 *         dependency path.
+		 */
+		if (lock->parent) { /* Parent exists, check prev dependency */
+			u8 dep = lock->dep;
+			bool prev_only_xr = lock->parent->only_xr;
 
 			/*
 			 * Mask out all -(S*)-> if we only have *R in previous
@@ -1701,26 +1699,49 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
 				continue;
 
 			/* If there are only -(*R)-> left, set that for the next step */
-			entry->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
+			lock->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
+		}
 
+		/*
+		 * Step 3: we haven't visited this and there is a strong
+		 *         dependency path to this, so check with @match.
+		 */
+		if (match(lock, data)) {
+			*target_entry = lock;
+			return BFS_RMATCH;
+		}
+
+		/*
+		 * Step 4: if not match, expand the path by adding the
+		 *         forward or backwards dependencis in the search
+		 *
+		 */
+		first = true;
+		head = get_dep_list(lock, offset);
+		list_for_each_entry_rcu(entry, head, entry) {
 			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;
-			}
+			/*
+			 * Note we only enqueue the first of the list into the
+			 * queue, because we can always find a sibling
+			 * dependency from one (see __bfs_next()), as a result
+			 * the space of queue is saved.
+			 */
+			if (!first)
+				continue;
+
+			first = false;
+
+			if (__cq_enqueue(cq, entry))
+				return BFS_EQUEUEFULL;
+
 			cq_depth = __cq_get_elem_count(cq);
 			if (max_bfs_queue_depth < cq_depth)
 				max_bfs_queue_depth = cq_depth;
 		}
 	}
-exit:
-	return ret;
+
+	return BFS_RNOMATCH;
 }
 
 static inline enum bfs_result