Re: [PATCH 06/14] net: sched: implement reference counted action release

Jiri Pirko
2018-05-15T09:04:02Z

Mon, May 14, 2018 at 09:07:06PM CEST, vladbu@mellanox.com wrote:
>
>On Mon 14 May 2018 at 16:47, Jiri Pirko <jiri@resnulli.us> wrote:
>> Mon, May 14, 2018 at 04:27:07PM CEST, vladbu@mellanox.com wrote:
>>
>> [...]
>>
>>
>>>+static int tcf_action_del_1(struct net *net, char *kind, u32 index,
>>>+ struct netlink_ext_ack *extack)
>>>+{
>>>+ const struct tc_action_ops *ops;
>>>+ int err = -EINVAL;
>>>+
>>>+ ops = tc_lookup_action_n(kind);
>>>+ if (!ops) {
>>
>> How this can happen? Apparently you hold reference to the module since
>> you put it couple lines below. In that case, I don't really understand
>> why you need to lookup and just don't use "ops" pointer you have in
>> tcf_action_delete().
>
>The problem is that I cant really delete action while holding reference

Wait a sec. I was talking about a "module reference" (module_put())

>to it. I can try to decrement reference twice, however that might result
>race condition if another task tries to delete that action at the same
>time.
>
>Imagine situation:
>1. Action is in actions idr, refcount==1
>2. Task tries to delete action, takes reference while working with it,
>refcount==2
>3. Another task tries to delete same action, takes reference,
>refcount==3
>4. First task decrements refcount twice, refcount==1
>5. At the same time second task decrements refcount twice, refcount==-1
>
>My solution is to save action index, but release the reference. Then try
>to lookup action again and delete it if it is still in idr. (was not
>concurrently deleted)
>
>Now same potential race condition with this patch:
>1. Action is in actions idr, refcount==1
>2. Task tries to delete action, takes reference while working with it,
>refcount==2
>3. Another task tries to delete same action, takes reference,
>refcount==3
>4. First task releases reference and deletes actions from idr, which
>results another refcount decrement, refcount==1
>5. At the same time second task releases reference to action,
>refcount==0, action is deleted
>6. When task tries to lookup-delete action from idr by index, action is
>not found. This case is handled correctly by code and no negative
>overflow of refcount happens.
>
>>
>>
>>>+ NL_SET_ERR_MSG(extack, "Specified TC action not found");
>>>+ goto err_out;
>>>+ }
>>>+
>>>+ if (ops->delete)
>>>+ err = ops->delete(net, index);
>>>+
>>>+ module_put(ops->owner);
>>>+err_out:
>>>+ return err;
>>>+}
>>>+
>>> static int tca_action_flush(struct net *net, struct nlattr *nla,
>>> struct nlmsghdr *n, u32 portid,
>>> struct netlink_ext_ack *extack)
>>>@@ -1052,6 +1088,36 @@ static int tca_action_flush(struct net *net, struct nlattr *nla,
>>> return err;
>>> }
>>>
>>>+static int tcf_action_delete(struct net *net, struct list_head *actions,
>>>+ struct netlink_ext_ack *extack)
>>>+{
>>>+ int ret;
>>>+ struct tc_action *a, *tmp;
>>>+ char kind[IFNAMSIZ];
>>>+ u32 act_index;
>>>+
>>>+ list_for_each_entry_safe(a, tmp, actions, list) {
>>>+ const struct tc_action_ops *ops = a->ops;
>>>+
>>>+ /* Actions can be deleted concurrently
>>>+ * so we must save their type and id to search again
>>>+ * after reference is released.
>>>+ */
>>>+ strncpy(kind, a->ops->kind, sizeof(kind) - 1);
>>>+ act_index = a->tcfa_index;
>>>+
>>>+ list_del(&a->list);
>>>+ if (tcf_action_put(a))
>>>+ module_put(ops->owner);
>>>+
>>>+ /* now do the delete */
>>>+ ret = tcf_action_del_1(net, kind, act_index, extack);
>>>+ if (ret < 0)
>>>+ return ret;
>>>+ }
>>>+ return 0;
>>>+}
>>>+
>>
>> [...]
>