Re: [PATCH v2] kref: warn on uninitialized kref

[Bart Van Assche <bvanassche@acm.org>]

On 05/17/14 14:38, Mikulas Patocka wrote:
> I found a memory leak in iSCSI target that was caused by kref initialized
> to zero (the memory object was allocated with kzalloc, kref_init was not
> called and kref_put_spinlock_irqsave was called which changed "0" to "-1"
> and didn't free the object).
> 
> Similar bugs may exist in other kernel areas, so I submit this patch that
> adds a check to kref.h. If the value is zero or negative, we can assume
> that it is uninitialized and we warn about it.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  include/linux/kref.h |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Index: linux-3.15-rc5/include/linux/kref.h
> ===================================================================
> --- linux-3.15-rc5.orig/include/linux/kref.h	2014-05-16 19:00:18.000000000 +0200
> +++ linux-3.15-rc5/include/linux/kref.h	2014-05-17 13:19:31.000000000 +0200
> @@ -69,7 +69,7 @@ static inline int kref_sub(struct kref *
>  	     void (*release)(struct kref *kref))
>  {
>  	WARN_ON(release == NULL);
> -
> +	WARN_ON_ONCE(atomic_read(&kref->refcount) < (int)count);
>  	if (atomic_sub_and_test((int) count, &kref->refcount)) {
>  		release(kref);
>  		return 1;
> @@ -119,6 +119,7 @@ static inline int kref_put_spinlock_irqs
>  	unsigned long flags;
>  
>  	WARN_ON(release == NULL);
> +	WARN_ON_ONCE(atomic_read(&kref->refcount) <= 0);
>  	if (atomic_add_unless(&kref->refcount, -1, 1))
>  		return 0;
>  	spin_lock_irqsave(lock, flags);
> @@ -136,6 +137,7 @@ static inline int kref_put_mutex(struct 
>  				 struct mutex *lock)
>  {
>  	WARN_ON(release == NULL);
> +	WARN_ON_ONCE(atomic_read(&kref->refcount) <= 0);
>  	if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
>  		mutex_lock(lock);
>  		if (unlikely(!atomic_dec_and_test(&kref->refcount))) {

This patch adds two conditional branches and one atomic read to
kref_sub(). What is the performance impact of this patch on kernel code
that uses kref_put() in the hot path ? Has it been considered to enable
the newly added code only if a CONFIG_DEBUG_* macro has been set ?

Bart.