lockdep: Show subclass in pretty print of lockdep output

The pretty print of the lockdep debug splat uses just the lock name
to show how the locking scenario happens. But when it comes to
nesting locks, the output becomes confusing which takes away the point
of the pretty printing of the lock scenario.

Without displaying the subclass info, we get the following output:

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(slock-AF_INET);
                                lock(slock-AF_INET);
                                lock(slock-AF_INET);
   lock(slock-AF_INET);

  *** DEADLOCK ***

The above looks more of a A->A locking bug than a A->B B->A.
By adding the subclass to the output, we can see what really happened:

 other info that might help us debug this:

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(slock-AF_INET);
                                lock(slock-AF_INET/1);
                                lock(slock-AF_INET);
   lock(slock-AF_INET/1);

  *** DEADLOCK ***

This bug was discovered while tracking down a real bug caught by lockdep.

Link: http://lkml.kernel.org/r/20111025202049.GB25043@hostway.ca
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Simon Kirby <sim@hostway.ca>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 91d67ce3a8d520a5cdc43d7abe6534c59710e79d..6bd915df5fd34871134f59c136e5fc7d8786bc48 100644 (file)
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -490,36 +490,32 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
        usage[i] = '\0';
 }
 
-static int __print_lock_name(struct lock_class *class)
+static void __print_lock_name(struct lock_class *class)
 {
        char str[KSYM_NAME_LEN];
        const char *name;
 
-       name = class->name;
-       if (!name)
-               name = __get_key_name(class->key, str);
-
-       return printk("%s", name);
-}
-
-static void print_lock_name(struct lock_class *class)
-{
-       char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
-       const char *name;
-
-       get_usage_chars(class, usage);
-
        name = class->name;
        if (!name) {
                name = __get_key_name(class->key, str);
-               printk(" (%s", name);
+               printk("%s", name);
        } else {
-               printk(" (%s", name);
+               printk("%s", name);
                if (class->name_version > 1)
                        printk("#%d", class->name_version);
                if (class->subclass)
                        printk("/%d", class->subclass);
        }
+}
+
+static void print_lock_name(struct lock_class *class)
+{
+       char usage[LOCK_USAGE_CHARS];
+
+       get_usage_chars(class, usage);
+
+       printk(" (");
+       __print_lock_name(class);
        printk("){%s}", usage);
 }