linux-kernel - [PATCH 05/10] lockdep: Show subclass in pretty print of lockdep output

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Date:	Mon, 07 Nov 2011 20:29:44 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH 05/10] lockdep: Show subclass in pretty print of lockdep output

From: Steven Rostedt <srostedt@...hat.com>

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@...llo.nl>
Reported-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Simon Kirby <sim@...tway.ca>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 kernel/lockdep.c |   30 +++++++++++++-----------------
 1 files changed, 13 insertions(+), 17 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 91d67ce..6bd915d 100644
--- 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);
 }
 
-- 
1.7.7



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