lockdep: Print the shortest dependency chain if finding a circle

Currently lockdep will print the 1st circle detected if it
exists when acquiring a new (next) lock.

This patch prints the shortest path from the next lock to be
acquired to the previous held lock if a circle is found.

The patch still uses the current method to check circle, and
once the circle is found, breadth-first search algorithem is
used to compute the shortest path from the next lock to the
previous lock in the forward lock dependency graph.

Printing the shortest path will shorten the dependency chain,
and make troubleshooting for possible circular locking easier.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1246201486-7308-2-git-send-email-tom.leiming@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 83 insertions, 0 deletions

diff --git a/kernel/lockdep_internals.h b/kernel/lockdep_internals.h
index 699a2ac3a0d..6f48d37d5be 100644
--- a/kernel/lockdep_internals.h
+++ b/kernel/lockdep_internals.h
@@ -136,3 +136,86 @@ extern atomic_t nr_find_usage_backwards_recursions;
 # define debug_atomic_dec(ptr)		do { } while (0)
 # define debug_atomic_read(ptr)		0
 #endif
+
+/* 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.
+ * */
+#define  MAX_CIRCULAR_QUE_SIZE	    4096UL
+struct circular_queue{
+	unsigned long element[MAX_CIRCULAR_QUE_SIZE];
+	unsigned int  front, rear;
+};
+
+#define LOCK_ACCESSED 		1UL
+#define LOCK_ACCESSED_MASK	(~LOCK_ACCESSED)
+
+static inline void __cq_init(struct circular_queue *cq)
+{
+	cq->front = cq->rear = 0;
+}
+
+static inline int __cq_empty(struct circular_queue *cq)
+{
+	return (cq->front == cq->rear);
+}
+
+static inline int __cq_full(struct circular_queue *cq)
+{
+	return ((cq->rear + 1)%MAX_CIRCULAR_QUE_SIZE)  == cq->front;
+}
+
+static inline int __cq_enqueue(struct circular_queue *cq, unsigned long elem)
+{
+	if (__cq_full(cq))
+		return -1;
+
+	cq->element[cq->rear] = elem;
+	cq->rear = (cq->rear + 1)%MAX_CIRCULAR_QUE_SIZE;
+	return 0;
+}
+
+static inline int __cq_dequeue(struct circular_queue *cq, unsigned long *elem)
+{
+	if (__cq_empty(cq))
+		return -1;
+
+	*elem = cq->element[cq->front];
+	cq->front = (cq->front + 1)%MAX_CIRCULAR_QUE_SIZE;
+	return 0;
+}
+
+static inline int __cq_get_elem_count(struct circular_queue *cq)
+{
+	return (cq->rear - cq->front)%MAX_CIRCULAR_QUE_SIZE;
+}
+
+static inline void mark_lock_accessed(struct lock_list *lock,
+					struct lock_list *parent)
+{
+	lock->parent = (void *) parent + LOCK_ACCESSED;
+}
+
+static inline unsigned long lock_accessed(struct lock_list *lock)
+{
+	return (unsigned long)lock->parent & LOCK_ACCESSED;
+}
+
+static inline struct lock_list *get_lock_parent(struct lock_list *child)
+{
+	return (struct lock_list *)
+		((unsigned long)child->parent & LOCK_ACCESSED_MASK);
+}
+
+static inline unsigned long get_lock_depth(struct lock_list *child)
+{
+	unsigned long depth = 0;
+	struct lock_list *parent;
+
+	while ((parent = get_lock_parent(child))) {
+		child = parent;
+		depth++;
+	}
+	return depth;
+}