lockdep: Print a nicer description for normal deadlocks
The lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a normal deadlock
scenario is detected by lockdep (lock A -> lock B and there
exists a place where lock B -> lock A) we now get the following
new output:
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(lockB);
lock(lockA);
lock(lockB);
lock(lockA);
*** DEADLOCK ***
On cases where there's a deeper chair, it shows the partial
chain that can cause the issue:
Chain exists of:
lockC --> lockA --> lockB
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(lockB);
lock(lockA);
lock(lockB);
lock(lockC);
*** DEADLOCK ***
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.380621789@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 7b2ffee..73cebd7 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1065,6 +1065,56 @@
return 0;
}
+static void
+print_circular_lock_scenario(struct held_lock *src,
+ struct held_lock *tgt,
+ struct lock_list *prt)
+{
+ struct lock_class *source = hlock_class(src);
+ struct lock_class *target = hlock_class(tgt);
+ struct lock_class *parent = prt->class;
+
+ /*
+ * A direct locking problem where unsafe_class lock is taken
+ * directly by safe_class lock, then all we need to show
+ * is the deadlock scenario, as it is obvious that the
+ * unsafe lock is taken under the safe lock.
+ *
+ * But if there is a chain instead, where the safe lock takes
+ * an intermediate lock (middle_class) where this lock is
+ * not the same as the safe lock, then the lock chain is
+ * used to describe the problem. Otherwise we would need
+ * to show a different CPU case for each link in the chain
+ * from the safe_class lock to the unsafe_class lock.
+ */
+ if (parent != source) {
+ printk("Chain exists of:\n ");
+ __print_lock_name(source);
+ printk(" --> ");
+ __print_lock_name(parent);
+ printk(" --> ");
+ __print_lock_name(target);
+ printk("\n\n");
+ }
+
+ printk(" Possible unsafe locking scenario:\n\n");
+ printk(" CPU0 CPU1\n");
+ printk(" ---- ----\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(");\n");
+ printk(" lock(");
+ __print_lock_name(parent);
+ printk(");\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(");\n");
+ printk(" lock(");
+ __print_lock_name(source);
+ printk(");\n");
+ printk("\n *** DEADLOCK ***\n\n");
+}
+
/*
* When a circular dependency is detected, print the
* header first:
@@ -1108,6 +1158,7 @@
{
struct task_struct *curr = current;
struct lock_list *parent;
+ struct lock_list *first_parent;
int depth;
if (!debug_locks_off_graph_unlock() || debug_locks_silent)
@@ -1121,6 +1172,7 @@
print_circular_bug_header(target, depth, check_src, check_tgt);
parent = get_lock_parent(target);
+ first_parent = parent;
while (parent) {
print_circular_bug_entry(parent, --depth);
@@ -1128,6 +1180,9 @@
}
printk("\nother info that might help us debug this:\n\n");
+ print_circular_lock_scenario(check_src, check_tgt,
+ first_parent);
+
lockdep_print_held_locks(curr);
printk("\nstack backtrace:\n");