Re: [PATCH net-next v6 00/11] Modify action API for implementing lockless actions

[Vlad Buslov <vladbu@mellanox.com>]

On Fri 13 Jul 2018 at 03:54, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> On Sat, Jul 7, 2018 at 8:43 PM David Miller <davem@davemloft.net> wrote:
>>
>> From: Vlad Buslov <vladbu@mellanox.com>
>> Date: Thu,  5 Jul 2018 17:24:22 +0300
>>
>> > Currently, all netlink protocol handlers for updating rules, actions and
>> > qdiscs are protected with single global rtnl lock which removes any
>> > possibility for parallelism. This patch set is a first step to remove
>> > rtnl lock dependency from TC rules update path.
>>  ...
>>
>> I'll apply this for now, I reviewed it a few more times and I see
>> where you are going with this.
>
> Dear David,
>
> I don't understand why you even believe the claim of lockless
> updaters here, it at least should raise a red flag when you see any
> kinda of this claim.
>
> I know you don't trust me, how about thinking it in this way:
>
> Why does RCU still require a lock for RCU writers? (Or at least
> RCU recommends a lock, if anyone really wants to point out some
> lockless algorithm here.)
>
> or:
>
> If writers could really go lockless as easily as Vlad claims, how could
> even Paul E. McKenney never bring it into RCU?
>
> Maybe Vlad is much cleverer than any of us here, and maybe he really
> discovers a very brilliant algorithm to allow TC actions to be updated
> locklessly, why not wait until he shows a proof (either code or a paper)?
> Is there a rush? I don't see it.
>
> In fact, I discussed this with Vlad a little bit at netdev TC workshop.
> I never see any brilliant algorithm from him from his slides, and I was
> told by him he used "copy and replace" to archive parallel updaters, I
> told him that is basically how RCU works and RCU writers have to be
> sync'ed with a lock (or at least recommended).
>
> Also, to confirm my judgement, I checked this with Paul privately too.
> Paul said you have to be extremely careful to go lockless, it is very hard
> to be bug free for lockless, although he _never_ says it is impossible.
>
> My _personal_ bet is that, lockless updates for TC filters or actions
> are impossible unless there are more things hiding behind "copy and
> replace", for example, some brilliant lockless algorithm. If lockless is
> really impossible in this circumstance, then many of your efforts in
> this patchset are vain, by the way.
>
> I _do_ believe you can break RTNL down to per device, per filter or per
> action, but no matter how small the locking scope is, there is still a lock.
> With a lock, there is no need to make things friendly to lockless, like
> making an integer increment inside an action to be atomic (your patch
> 02/11).
>
> Please _do_ prove my personal judgement is wrong, by showing your
> final code or a formal paper/article. I am very *happy* to be proved
> to be wrong here, I am very open to change my mind here.
>
> Vlad, we need your proof. Please prove I am wrong, seriously!!! :)
>
> Thanks to anyone for proving me I am wrong just in case!!! :)

Dear Cong,

I never claimed to have some new brilliant algorithm that completely
removed any locks from rules update path. Obviously, fine-grained
locking is introduced when necessary. I'm sorry if my liberal usage of
term "lockless" confused you. I guess I should be more specific. I'm
fully agree with you that totally removing any and all locks from rules
update path would require some engineering marvel.