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



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