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, 106 insertions, 9 deletions

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 8bbeef996c76..93dc70d18cdf 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -897,6 +897,79 @@ static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
         return 1;
 }
 
+static struct circular_queue  lock_cq;
+static int __search_shortest_path(struct lock_list *source_entry,
+                                struct lock_class *target,
+                                struct lock_list **target_entry,
+                                int forward)
+{
+        struct lock_list *entry;
+        struct circular_queue *cq = &lock_cq;
+        int ret = 1;
+
+        __cq_init(cq);
+
+        mark_lock_accessed(source_entry, NULL);
+        if (source_entry->class == target) {
+                *target_entry = source_entry;
+                ret = 0;
+                goto exit;
+        }
+
+        __cq_enqueue(cq, (unsigned long)source_entry);
+
+        while (!__cq_empty(cq)) {
+                struct lock_list *lock;
+                struct list_head *head;
+
+                __cq_dequeue(cq, (unsigned long *)&lock);
+
+                if (!lock->class) {
+                        ret = -2;
+                        goto exit;
+                }
+
+                if (forward)
+                        head = &lock->class->locks_after;
+                else
+                        head = &lock->class->locks_before;
+
+                list_for_each_entry(entry, head, entry) {
+                        if (!lock_accessed(entry)) {
+                                mark_lock_accessed(entry, lock);
+                                if (entry->class == target) {
+                                        *target_entry = entry;
+                                        ret = 0;
+                                        goto exit;
+                                }
+
+                                if (__cq_enqueue(cq, (unsigned long)entry)) {
+                                        ret = -1;
+                                        goto exit;
+                                }
+                        }
+                }
+        }
+exit:
+        return ret;
+}
+
+static inline int __search_forward_shortest_path(struct lock_list *src_entry,
+                                struct lock_class *target,
+                                struct lock_list **target_entry)
+{
+        return __search_shortest_path(src_entry, target, target_entry, 1);
+
+}
+
+static inline int __search_backward_shortest_path(struct lock_list *src_entry,
+                                struct lock_class *target,
+                                struct lock_list **target_entry)
+{
+        return __search_shortest_path(src_entry, target, target_entry, 0);
+
+}
+
 /*
  * Recursive, forwards-direction lock-dependency checking, used for
  * both noncyclic checking and for hardirq-unsafe/softirq-unsafe
@@ -934,7 +1007,7 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth)
 {
         struct task_struct *curr = current;
 
-        if (!debug_locks_off_graph_unlock() || debug_locks_silent)
+        if (debug_locks_silent)
                 return 0;
 
         printk("\n=======================================================\n");
@@ -954,19 +1027,41 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth)
         return 0;
 }
 
-static noinline int print_circular_bug_tail(void)
+static noinline int print_circular_bug(void)
 {
         struct task_struct *curr = current;
         struct lock_list this;
+        struct lock_list *target;
+        struct lock_list *parent;
+        int result;
+        unsigned long depth;
 
-        if (debug_locks_silent)
+        if (!debug_locks_off_graph_unlock() || debug_locks_silent)
                 return 0;
 
         this.class = hlock_class(check_source);
         if (!save_trace(&this.trace))
                 return 0;
 
-        print_circular_bug_entry(&this, 0);
+        result = __search_forward_shortest_path(&this,
+                                                hlock_class(check_target),
+                                                &target);
+        if (result) {
+                printk("\n%s:search shortest path failed:%d\n", __func__,
+                        result);
+                return 0;
+        }
+
+        depth = get_lock_depth(target);
+
+        print_circular_bug_header(target, depth);
+
+        parent = get_lock_parent(target);
+
+        while (parent) {
+                print_circular_bug_entry(parent, --depth);
+                parent = get_lock_parent(parent);
+        }
 
         printk("\nother info that might help us debug this:\n\n");
         lockdep_print_held_locks(curr);
@@ -1072,14 +1167,15 @@ check_noncircular(struct lock_class *source, unsigned int depth)
          */
         list_for_each_entry(entry, &source->locks_after, entry) {
                 if (entry->class == hlock_class(check_target))
-                        return print_circular_bug_header(entry, depth+1);
+                        return 2;
                 debug_atomic_inc(&nr_cyclic_checks);
-                if (!check_noncircular(entry->class, depth+1))
-                        return print_circular_bug_entry(entry, depth+1);
+                if (check_noncircular(entry->class, depth+1) == 2)
+                        return 2;
         }
         return 1;
 }
 
+
 #if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
 /*
  * Forwards and backwards subgraph searching, for the purposes of
@@ -1484,8 +1580,9 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
          */
         check_source = next;
         check_target = prev;
-        if (!(check_noncircular(hlock_class(next), 0)))
-                return print_circular_bug_tail();
+        if (check_noncircular(hlock_class(next), 0) == 2)
+                return print_circular_bug();
+
 
         if (!check_prev_add_irq(curr, prev, next))
                 return 0;