On Fri, Jan 04, 2019 at 10:29:13PM +0800, zhe...@windriver.com wrote:
> It's not necessary to keep consistency between readers and writers of
> kmemleak_lock. RCU is more proper for this case. And in order to gain better
> performance, we turn the reader locks to RCU read locks and writer locks to
> normal spin locks.
This won't work.
> @@ -515,9 +515,7 @@ static struct kmemleak_object
> *find_and_get_object(unsigned long ptr, int alias)
> struct kmemleak_object *object;
>
> rcu_read_lock();
> - read_lock_irqsave(&kmemleak_lock, flags);
> object = lookup_object(ptr, alias);
> - read_unlock_irqrestore(&kmemleak_lock, flags);
The comment on lookup_object() states that the kmemleak_lock must be
held. That's because we don't have an RCU-like mechanism for removing
removing objects from the object_tree_root:
>
> /* check whether the object is still available */
> if (object && !get_object(object))
> @@ -537,13 +535,13 @@ static struct kmemleak_object
> *find_and_remove_object(unsigned long ptr, int ali
> unsigned long flags;
> struct kmemleak_object *object;
>
> - write_lock_irqsave(&kmemleak_lock, flags);
> + spin_lock_irqsave(&kmemleak_lock, flags);
> object = lookup_object(ptr, alias);
> if (object) {
> rb_erase(&object->rb_node, &object_tree_root);
> list_del_rcu(&object->object_list);
> }
> - write_unlock_irqrestore(&kmemleak_lock, flags);
> + spin_unlock_irqrestore(&kmemleak_lock, flags);
So here, while list removal is RCU-safe, rb_erase() is not.
If you have time to implement an rb_erase_rcu(), than we could reduce
the locking in kmemleak.
--
Catalin
