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    SubjectRe: [PATCH v2] kref: warn on uninitialized kref
    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.



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