Subject: Re: [PATCH 10/14] net: sched: extend act API for lockless actions

From: Jiri Pirko <[email-masked]>
Date: Wed, 16 May 2018 10:56:28 +0200

Wed, May 16, 2018 at 10:16:13AM CEST, [email-masked] wrote:
>
>On Wed 16 May 2018 at 07:50, Jiri Pirko <[email-masked]> wrote:
>> Mon, May 14, 2018 at 04:27:11PM CEST, [email-masked] wrote:
>>>Implement new action API function to atomically delete action with
>>>specified index and to atomically insert unique action. These functions are
>>>required to implement init and delete functions for specific actions that
>>>do not rely on rtnl lock.
>>>
>>>Signed-off-by: Vlad Buslov <[email-masked]>
>>>---
>>> include/net/act_api.h |  2 ++
>>> net/sched/act_api.c   | 45 +++++++++++++++++++++++++++++++++++++++++++++
>>> 2 files changed, 47 insertions(+)
>>>
>>>diff --git a/include/net/act_api.h b/include/net/act_api.h
>>>index a8c8570..bce0cf1 100644
>>>--- a/include/net/act_api.h
>>>+++ b/include/net/act_api.h
>>>@@ -153,7 +153,9 @@ int tcf_idr_create(struct tc_action_net *tn, u32 index, struct nlattr *est,
>>> 		   struct tc_action **a, const struct tc_action_ops *ops,
>>> 		   int bind, bool cpustats);
>>> void tcf_idr_insert(struct tc_action_net *tn, struct tc_action *a);
>>>+void tcf_idr_insert_unique(struct tc_action_net *tn, struct tc_action *a);
>>> 
>>>+int tcf_idr_find_delete(struct tc_action_net *tn, u32 index);
>>> int __tcf_idr_release(struct tc_action *a, bool bind, bool strict);
>>> 
>>> static inline int tcf_idr_release(struct tc_action *a, bool bind)
>>>diff --git a/net/sched/act_api.c b/net/sched/act_api.c
>>>index 2772276e..a5193dc 100644
>>>--- a/net/sched/act_api.c
>>>+++ b/net/sched/act_api.c
>>>@@ -330,6 +330,41 @@ bool tcf_idr_check(struct tc_action_net *tn, u32 index, struct tc_action **a,
>>> }
>>> EXPORT_SYMBOL(tcf_idr_check);
>>> 
>>>+int tcf_idr_find_delete(struct tc_action_net *tn, u32 index)
>>>+{
>>>+	struct tcf_idrinfo *idrinfo = tn->idrinfo;
>>>+	struct tc_action *p;
>>>+	int ret = 0;
>>>+
>>>+	spin_lock_bh(&idrinfo->lock);
>>
>> Why "_bh" is needed here?
>
>Original idr remove function used _bh version so I used it here as well.
>As I already replied to your previous question about idrinfo lock usage,
>I don't see any particular reason for locking with _bh at this point.
>I've contacted the author(Chris Mi) and he said that he just preserved
>locking the same way as it was before he changed hash table to idr for
>action lookup.
>
>You want me to do standalone patch that cleans up idrinfo locking?

Yes please. You can send it separately, not as a part of this patchset.

>
>>
>>
>>>+	p = idr_find(&idrinfo->action_idr, index);
>>>+	if (!p) {
>>>+		spin_unlock(&idrinfo->lock);
>>>+		return -ENOENT;
>>>+	}
>>>+
>>>+	if (!atomic_read(&p->tcfa_bindcnt)) {
>>>+		if (refcount_dec_and_test(&p->tcfa_refcnt)) {
>>>+			struct module *owner = p->ops->owner;
>>>+
>>>+			WARN_ON(p != idr_remove(&idrinfo->action_idr,
>>>+						p->tcfa_index));
>>>+			spin_unlock_bh(&idrinfo->lock);
>>>+
>>>+			tcf_action_cleanup(p);
>>>+			module_put(owner);
>>>+			return 0;
>>>+		}
>>>+		ret = 0;
>>>+	} else {
>>>+		ret = -EPERM;
>>
>> I wonder if "-EPERM" is the best error code for this...
>
>This is what original code returned so I decided to preserve
>compatibility.

Okay.

>
>>
>>
>>>+	}
>>>+
>>>+	spin_unlock_bh(&idrinfo->lock);
>>>+	return ret;
>>>+}
>>>+EXPORT_SYMBOL(tcf_idr_find_delete);
>>>+
>>> int tcf_idr_create(struct tc_action_net *tn, u32 index, struct nlattr *est,
>>> 		   struct tc_action **a, const struct tc_action_ops *ops,
>>> 		   int bind, bool cpustats)
>>>@@ -407,6 +442,16 @@ void tcf_idr_insert(struct tc_action_net *tn, struct tc_action *a)
>>> }
>>> EXPORT_SYMBOL(tcf_idr_insert);
>>> 
>>>+void tcf_idr_insert_unique(struct tc_action_net *tn, struct tc_action *a)
>>>+{
>>>+	struct tcf_idrinfo *idrinfo = tn->idrinfo;
>>>+
>>>+	spin_lock_bh(&idrinfo->lock);
>>>+	WARN_ON(idr_replace(&idrinfo->action_idr, a, a->tcfa_index));
>>
>> Under which condition this WARN_ON is hit?
>
>When idr replace returns non-NULL pointer, which means that somehow
>concurrent insertion of action with same index has happened and we are
>leaking memory.

Is that possible to happen? Meaning, can I as a user cause this by doing
something in a wrong/unexpected way?

>
>By the way I'm still not sure if having this insert unique function is
>warranted or I should just add WARN to regular idr insert. What is your
>opinion on this?

I have to check where you use this.

>
>>
>>
>>>+	spin_unlock_bh(&idrinfo->lock);
>>>+}
>>>+EXPORT_SYMBOL(tcf_idr_insert_unique);
>>>+
>>> void tcf_idrinfo_destroy(const struct tc_action_ops *ops,
>>> 			 struct tcf_idrinfo *idrinfo)
>>> {
>>>-- 
>>>2.7.5
>>>
>