On 1/23/23 14:24, Catalin Marinas wrote:
On Fri, Jan 20, 2023 at 05:54:28PM -0500, Waiman Long wrote:
On 1/20/23 14:18, Catalin Marinas wrote:
/*
@@ -633,6 +642,7 @@ static void __create_object(unsigned long ptr, size_t size,
object->count = 0; /* white color initially */
object->jiffies = jiffies;
object->checksum = 0;
+ object->del_state = 0;
/* task information */
if (in_hardirq()) {
@@ -1470,9 +1480,22 @@ static void kmemleak_cond_resched(struct kmemleak_object *object)
if (!get_object(object))
return; /* Try next object */
+ raw_spin_lock_irq(&kmemleak_lock);
+ if (object->del_state & DELSTATE_REMOVED)
+ goto unlock_put; /* Object removed */
+ object->del_state |= DELSTATE_NO_DELETE;
+ raw_spin_unlock_irq(&kmemleak_lock);
+
rcu_read_unlock();
cond_resched();
rcu_read_lock();
+
+ raw_spin_lock_irq(&kmemleak_lock);
+ if (object->del_state & DELSTATE_REMOVED)
+ list_del_rcu(&object->object_list);
+ object->del_state &= ~DELSTATE_NO_DELETE;
+unlock_put:
+ raw_spin_unlock_irq(&kmemleak_lock);
put_object(object);
}
I'm not sure this was the only problem. We do have the problem that the
current object may be removed from the list, solved above, but another
scenario I had in mind is the next object being released during this
brief resched period. The RCU relies on object->next->next being valid
but, with a brief rcu_read_unlock(), the object->next could be freed,
reallocated, so object->next->next invalid.
Looking at the following scenario,
object->next => A (removed)
A->next => B (removed)
As object->next is pointing to A, A must still be allocated and not freed
yet. Now if B is also removed, there are 2 possible case.
1) B is removed from the list after the removal of A. In that case, it is
not possible that A is allocated, but B is freed.
2) B is removed before A. A->next can't pointed to B when it is being
removed. Due to weak memory ordering, it is possible that another cpu can
see A->next still pointing to B. In that case, I believe that it is still
within the grace period where neither A or B is freed.
In fact, it is no different from a regular scanning of the object list
without ever called cond_resched().
More like thinking out loud:
The lockless RCU loop relies on object->next->next being valid within
the grace period (A not freed). Due to weak memory ordering, the looping
CPU may not observe the object->next update (removal of A) by another
CPU, so it continues to loop over it. But since we do an
rcu_read_unlock() in the middle of the loop, I don't think these
assumptions are still valid, so A may be freed.
What we need is that object->next reading for the following iteration
either sees the updated object->next (B) or it sees A but the latter
still around. I think this holds with the proposed
kmemleak_cond_resched() since we now start a new grace period with
rcu_read_lock() followed by taking and releasing kmemleak_lock. The
latter would give us the memory ordering required since removing object
A from the list does take the lock.
So yeah, you are probably right, I just find it hard to get my head
around ;). I still think it would be simpler with a single kmemleak_lock
(no object->lock) but that's more involved than a simple fix.
Assuming your (and my) reasoning above is correct:
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
I should have mentioned the fact that taking the kmemleak_lock will post
some ordering guarantee since it is done after a new rcu_read_lock(). So
yes, even if both A and B are removed from the object_list, they should
still be around and not freed yet.
Thanks for your review.
Cheers,
Longman
