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, 6 insertions, 0 deletions

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index b25d1b53df0d..9ec026f8d09e 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -149,6 +149,12 @@ struct lock_list {
 	struct lock_class		*class;
 	struct stack_trace		trace;
 	int				distance;
+
+	/*The parent field is used to implement breadth-first search,and
+	 *the bit 0 is reused to indicate if the lock has been accessed
+	 *in BFS.
+	 */
+	struct lock_list		*parent;
 };
 
 /*