Re: [PATCH] tty: add lockdep annotations

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

On Sat, 2012-06-02 at 09:17 +0200, Eric Dumazet wrote:

> 
> Here is the patch I am currently testing (need to boot the machine ~50
> times to make sure it is the right fix)

Seems good, but am still doing reboots to make sure.

We should add a debugging code to kref to catch poisoned refcounts...

Something like :

[PATCH] kref: detects poisoned refcounts

Catch kref_get() done on freed memory, if slub/slab are poisoning the
freed area.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/kref.h   |    2 ++
 include/linux/poison.h |    3 +++
 2 files changed, 5 insertions(+)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 9c07dce..d8e90a1 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));
+	WARN_ON(atomic_read(&kref->refcount) == POISON_FREE_INT);
 	atomic_inc(&kref->refcount);
 }
 
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