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

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Date:   Tue, 15 May 2018 11:03:57 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, pablo@...filter.org,
        kadlec@...ckhole.kfki.hu, fw@...len.de, ast@...nel.org,
        daniel@...earbox.net, edumazet@...gle.com, keescook@...omium.org,
        linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, kliteyn@...lanox.com
Subject: Re: [PATCH 06/14] net: sched: implement reference counted action
 release

Mon, May 14, 2018 at 09:07:06PM CEST, vladbu@...lanox.com wrote:
>
>On Mon 14 May 2018 at 16:47, Jiri Pirko <jiri@...nulli.us> wrote:
>> Mon, May 14, 2018 at 04:27:07PM CEST, vladbu@...lanox.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;
>>>+}
>>>+
>>
>> [...]
>