Re: [PATCH] tty: add lockdep annotations

[Eric Dumazet]

On Sat, 2012-06-02 at 12:57 +0100, Alan Cox wrote:

> I'll have a look at this ASAP - I think the driver ref counting side is a
> red herring but I'll build an Ubuntu 12 setup and try and replicate the
> crash once I'm back at work (its a national holiday here Sat-Tue for the
> English queens jubilee)

With following debugging patch I now have crashes at every boot, so it
might be more easy to find the bug :

stack trace :
  tty_lock  (BUG_ON in kref_get())
  tty_open
  chrdev_open
  do_dentry_open
  nameidata_to_filp

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 9c07dce..49a9acf 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -18,6 +18,7 @@
 #include <linux/bug.h>
 #include <linux/atomic.h>
 #include <linux/kernel.h>
+#include <linux/poison.h>
 
 struct kref {
 	atomic_t refcount;
@@ -39,6 +40,7 @@ static inline void kref_init(struct kref *kref)
 static inline void kref_get(struct kref *kref)
 {
 	WARN_ON(!atomic_read(&kref->refcount));
+	BUG_ON(atomic_read(&kref->refcount) == POISON_FREE_INT);
 	atomic_inc(&kref->refcount);
 }
 
@@ -65,7 +67,9 @@ static inline int kref_sub(struct kref *kref, unsigned int count,
 {
 	WARN_ON(release == NULL);
 
+	BUG_ON(atomic_read(&kref->refcount) == POISON_FREE_INT);
 	if (atomic_sub_and_test((int) count, &kref->refcount)) {
+		atomic_set(&kref->refcount, POISON_FREE_INT);
 		release(kref);
 		return 1;
 	}
diff --git a/include/linux/poison.h b/include/linux/poison.h
index 2110a81..2947582 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -48,6 +48,9 @@
 #define POISON_FREE	0x6b	/* for use-after-free poisoning */
 #define	POISON_END	0xa5	/* end-byte of poisoning */
 
+#define POISON_FREE_SHORT ((POISON_FREE << 8) + POISON_FREE)
+#define POISON_FREE_INT   ((POISON_FREE_SHORT << 16) + POISON_FREE_SHORT)
+
 /********** arch/$ARCH/mm/init.c **********/
 #define POISON_FREE_INITMEM	0xcc
 


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