locking/lockdep: Change type of the element field in circular_queue

The element field is an array in struct circular_queue to keep track of locks
in the search. Making it the same type as the locks avoids type cast. Also
fix a typo and elaborate the comment above struct circular_queue.

No functional change.
Signed-off-by: Yuyang Du <duyuyang@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: frederic@kernel.org
Cc: ming.lei@redhat.com
Cc: will.deacon@arm.com
Link: https://lkml.kernel.org/r/20190506081939.74287-13-duyuyang@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -1262,13 +1262,17 @@ static int add_lock_to_list(struct lock_class *this,
 #define CQ_MASK				(MAX_CIRCULAR_QUEUE_SIZE-1)
 
 /*
- * The circular_queue and helpers is used to implement the
- * breadth-first search(BFS)algorithem, by which we can build
- * the shortest path from the next lock to be acquired to the
- * previous held lock if there is a circular between them.
+ * The circular_queue and helpers are used to implement graph
+ * breadth-first search (BFS) algorithm, by which we can determine
+ * whether there is a path from a lock to another. In deadlock checks,
+ * a path from the next lock to be acquired to a previous held lock
+ * indicates that adding the <prev> -> <next> lock dependency will
+ * produce a circle in the graph. Breadth-first search instead of
+ * depth-first search is used in order to find the shortest (circular)
+ * path.
  */
 struct circular_queue {
-	unsigned long element[MAX_CIRCULAR_QUEUE_SIZE];
+	struct lock_list *element[MAX_CIRCULAR_QUEUE_SIZE];
 	unsigned int  front, rear;
 };
 
@@ -1294,7 +1298,7 @@ static inline int __cq_full(struct circular_queue *cq)
 	return ((cq->rear + 1) & CQ_MASK) == cq->front;
 }
 
-static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
+static inline int __cq_enqueue(struct circular_queue *cq, struct lock_list *elem)
 {
 	if (__cq_full(cq))
 		return -1;
@@ -1304,7 +1308,7 @@ static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
 	return 0;
 }
 
-static inline int __cq_dequeue(struct circular_queue *cq, unsigned long *elem)
+static inline int __cq_dequeue(struct circular_queue *cq, struct lock_list **elem)
 {
 	if (__cq_empty(cq))
 		return -1;
@@ -1382,12 +1386,12 @@ static int __bfs(struct lock_list *source_entry,
 		goto exit;
 
 	__cq_init(cq);
-	__cq_enqueue(cq, (unsigned long)source_entry);
+	__cq_enqueue(cq, source_entry);
 
 	while (!__cq_empty(cq)) {
 		struct lock_list *lock;
 
-		__cq_dequeue(cq, (unsigned long *)&lock);
+		__cq_dequeue(cq, &lock);
 
 		if (!lock->class) {
 			ret = -2;
@@ -1411,7 +1415,7 @@ static int __bfs(struct lock_list *source_entry,
 					goto exit;
 				}
 
-				if (__cq_enqueue(cq, (unsigned long)entry)) {
+				if (__cq_enqueue(cq, entry)) {
 					ret = -1;
 					goto exit;
 				}