[PATCH 3/7] lockdep: Print a nice description of simple deadlock

[Steven Rostedt <rostedt@goodmis.org>]

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From: Steven Rostedt <srostedt@redhat.com>

The lockdep output can be pretty cryptic, having a nice output
can save a lot of head scratching. When a simple deadlock scenario
is detected by lockdep (lock A -> lock A) we get the following output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(lock)->rlock);
  lock(&(lock)->rlock);

 *** DEADLOCK ***

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/lockdep.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 1039008..a0f253b 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1664,6 +1664,26 @@ static inline void inc_chains(void)
 
 #endif
 
+static void
+print_deadlock_scenario(struct held_lock *nxt,
+			     struct held_lock *prv)
+{
+	struct lock_class *next = hlock_class(nxt);
+	struct lock_class *prev = hlock_class(prv);
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0\n");
+	printk("       ----\n");
+	printk("  lock(");
+	__print_lock_name(prev);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(next);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+	printk(" May be due to missing lock nesting notation\n\n");
+}
+
 static int
 print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 		   struct held_lock *next)
@@ -1682,6 +1702,7 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
 	print_lock(prev);
 
 	printk("\nother info that might help us debug this:\n");
+	print_deadlock_scenario(next, prev);
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");
-- 
1.7.2.3