netdev - Re: [PATCH net-next 17/17] net: sched: unlock rules update API

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Date:   Wed, 14 Nov 2018 07:44:53 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH net-next 17/17] net: sched: unlock rules update API

Tue, Nov 13, 2018 at 02:46:54PM CET, vladbu@...lanox.com wrote:
>On Mon 12 Nov 2018 at 17:30, David Miller <davem@...emloft.net> wrote:
>> From: Vlad Buslov <vladbu@...lanox.com>
>> Date: Mon, 12 Nov 2018 09:55:46 +0200
>>
>>> Register netlink protocol handlers for message types RTM_NEWTFILTER,
>>> RTM_DELTFILTER, RTM_GETTFILTER as unlocked. Set rtnl_held variable that
>>> tracks rtnl mutex state to be false by default.
>>
>> This whole conditional locking mechanism is really not clean and makes
>> this code so much harder to understand and audit.
>>
>> Please improve the code so that this kind of construct is not needed.
>>
>> Thank you.
>
>Hi David,
>
>I considered several approaches to this problem and decided that this
>one is most straightforward to implement. I understand your concern and
>agree that this code is not easiest to understand and can suggest
>several possible solutions that do not require this kind of elaborate
>locking mechanism in cls API, but have their own drawbacks:
>
>1. Convert all qdiscs and classifiers to support unlocked execution,
>like we did for actions. However, according to my experience with
>converting flower classifier, these require much more code than actions.
>I would estimate it to be more work than whole current unlocking effort
>(hundred+ patches). Also, authors of some of them might be unhappy with
>such intrusive changes. I don't think this approach is realistic.
>
>2. Somehow determine if rtnl is needed at the beginning of cls API rule
>update functions. Currently, this is not possible because locking
>requirements are determined by qdisc_class_ops and tcf_proto_ops 'flags'
>field, which requires code to first do whole ops lookup sequence.
>However, instead of class field I can put 'flags' in some kind of hash
>table or array that will map qdisc/classifier type string to flags, so
>it will be possible to determine locking requirements by just parsing
>netlink message and obtaining flags by qdisc/classifier type. I do not
>consider it pretty solution either, but maybe you have different
>opinion.

I think you will have to do 2. or some modification. Can't you just
check for cls ability to run unlocked early on in tc_new_tfilter()?
You would call tcf_proto_locking_check(nla_data(tca[TCA_KIND]), ...),
which would do tcf_proto_lookup_ops() for ops and check the flags?


>
>3. Anything you can suggest? I might be missing something simple that
>you would consider more elegant solution to this problem.
>
>Thanks,
>Vlad
>