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From: Igor Ryzhov <iryzhov@nfware.com>
To: Elad Nachman <eladv6@gmail.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>,
	Ferruh Yigit <ferruh.yigit@intel.com>, dev <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v4
Date: Mon, 1 Mar 2021 11:10:01 +0300
Message-ID: <CAF+s_FwHiHs9TkS90UpLBtzowFo-c667nWVOF+y1p8Qn_-_5WQ@mail.gmail.com> (raw)
In-Reply-To: <CACXF7qk=Jozo9ahfmjX5yNdBvFBNeaEKCpCCq5KoyZVuyknbjA@mail.gmail.com>

Stephen,

No, I don't have a better proposal, but I think it is not correct to change
the behavior of KNI (making link down without a real response).
Even though we know that communicating with userspace under rtnl_lock is a
bad idea, it works as it is for many years already.

Elad,

I agree with you that KNI should be removed from the main tree if it is not
possible to fix this __dev_close_many issue.
There were discussions about this multiple times already, but no one is
working on this AFAIK.
Last time the discussion was a month ago:
https://www.mail-archive.com/dev@dpdk.org/msg196033.html

Igor

On Fri, Feb 26, 2021 at 8:43 PM Elad Nachman <eladv6@gmail.com> wrote:

> The way the kernel handles its locks and lists for the dev close many
> path, there is no way you can go around this with rtnl unlocked :
> "
>
> There is a race condition in __dev_close_many() processing the
> close_list while the application terminates.
> It looks like if two vEth devices are terminating,
> and one releases the rtnl lock, the other takes it,
> updating the close_list in an unstable state,
> causing the close_list to become a circular linked list,
> hence list_for_each_entry() will endlessly loop inside
> __dev_close_many() .
>
> "
> And I don't expect David Miller will bend the kernel networking for DPDK
> or KNI.
>
> But - Stephen - if you can personally convince David to accept a
> kernel patch which will separate the close_list locking mechanism to a
> separate (RCU?) lock, then I can introduce first a patch to the kernel
> which will add a lock for the close_list, this way rtnl can be
> unlocked for the if down case.
>
> After that kernel patch, your original patch + relocation of the sync
> mutex locking will do the job .
>
> Otherwise, rtnl has to be kept locked all of the way for the if down
> case in order to prevent corruption causing a circular linked list out
> of the close_list, causing a hang in the kernel.
>
> Currently, the rtnl lock is the only thing keeping the close_list from
> corruption.
>
> If you doubt rtnl cannot be unlocked for dev close path, you can
> consult David for his opinion, as I think it is critical to understand
> what the kernel can or cannot do, or expects to be done before we can
> unlock its locks as we wish inside rte_kni.ko .
>
> Otherwise, if we are still in disagreement on how to patch this set of
> problems, I think the responsible way around it is to completely
> remove kni from the main dpdk tree and move it to dpdk-kmods
> repository.
>
> I know BSD style open-source does not carry legal responsibility from
> the developers, but I think when a bunch of developers know a piece of
> code is highly buggy, they should not leave it for countless new users
> to bounce their head desperately against, if they cannot agree on a
> correct way to solve the bunch of problems, of which I think we all
> agree exist (we just do not agree on the proper solution or patch)...
>
> That's my two cents,
>
> Elad.
>
> On Fri, Feb 26, 2021 at 5:49 PM Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> >
> > On Fri, 26 Feb 2021 00:01:01 +0300
> > Igor Ryzhov <iryzhov@nfware.com> wrote:
> >
> > > Hi Elad,
> > >
> > > Thanks for the patch, but this is still NACK from me.
> > >
> > > The only real advantage of KNI over other exceptional-path techniques
> > > like virtio-user is the ability to configure DPDK-managed interfaces
> > > directly
> > > from the kernel using well-known utils like iproute2. A very important
> part
> > > of this is getting responses from the DPDK app and knowing the actual
> > > result of command execution.
> > > If you're making async requests to the application and you don't know
> > > the result, then what's the point of using KNI at all?
> > >
> > > Igor
> >
> > Do you have a better proposal that keeps the request result but does not
> > call userspace with lock held.
> >
> > PS: I also have strong dislike of KNI, as designed it would have been
> rejected
> > by Linux kernel developers.  A better solution would be userspace
> version of
> > something like devlink devices. But doing control operations by proxy is
> > a locking nightmare.
>

  reply	other threads:[~2021-03-01  8:10 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-26 14:46 [dpdk-dev] [PATCH] kni: fix rtnl deadlocks and race conditions Elad Nachman
2021-02-19 18:41 ` Ferruh Yigit
2021-02-21  8:03   ` Elad Nachman
2021-02-22 15:58     ` Ferruh Yigit
2021-02-23 12:05 ` [dpdk-dev] [PATCH V2] kni: fix rtnl deadlocks and race conditions v2 Elad Nachman
2021-02-23 12:53   ` Ferruh Yigit
2021-02-23 13:44 ` [dpdk-dev] [PATCH 1/2] kni: fix rtnl deadlocks and race conditions v3 Elad Nachman
2021-02-23 13:45 ` [dpdk-dev] [PATCH 2/2] " Elad Nachman
2021-02-24 12:49   ` Igor Ryzhov
2021-02-24 13:33     ` Elad Nachman
2021-02-24 14:04       ` Igor Ryzhov
2021-02-24 14:06         ` Elad Nachman
2021-02-24 14:41           ` Igor Ryzhov
2021-02-24 14:56             ` Elad Nachman
2021-02-24 15:18               ` Igor Ryzhov
     [not found]                 ` <CACXF7qkhkzFc-=v=iiBzh2V7rLjk1U34VUfPbNrnYJND_0TKHQ@mail.gmail.com>
2021-02-24 16:31                   ` Igor Ryzhov
2021-02-24 15:54     ` Stephen Hemminger
2021-02-25 14:32 ` [dpdk-dev] [PATCH 1/2] kni: fix kernel deadlock when using mlx devices Elad Nachman
2021-02-25 14:32   ` [dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v4 Elad Nachman
2021-02-25 21:01     ` Igor Ryzhov
2021-02-26 15:48       ` Stephen Hemminger
2021-02-26 17:43         ` Elad Nachman
2021-03-01  8:10           ` Igor Ryzhov [this message]
2021-03-01 16:38             ` Stephen Hemminger
2021-03-15 16:58               ` Ferruh Yigit
2021-03-01 20:27             ` Dan Gora
2021-03-01 21:26               ` Dan Gora
2021-03-02 16:44                 ` Elad Nachman
2021-03-15 17:17     ` Ferruh Yigit
2021-03-16 18:35       ` Elad Nachman
2021-03-16 18:42         ` Ferruh Yigit
2021-03-15 17:17   ` [dpdk-dev] [PATCH 1/2] kni: fix kernel deadlock when using mlx devices Ferruh Yigit
2021-03-29 14:36 ` [dpdk-dev] [PATCH v5 1/3] kni: refactor user request processing Ferruh Yigit
2021-03-29 14:36   ` [dpdk-dev] [PATCH v5 2/3] kni: support async user request Ferruh Yigit
2021-03-29 14:36   ` [dpdk-dev] [PATCH v5 3/3] kni: fix kernel deadlock when using mlx devices Ferruh Yigit
2021-04-09 14:56     ` [dpdk-dev] [dpdk-stable] " Ferruh Yigit
2021-04-12 14:35       ` Elad Nachman
2021-04-20 23:07         ` Thomas Monjalon
2021-04-23  8:41           ` Igor Ryzhov
2021-04-23  8:59             ` Ferruh Yigit
2021-04-23 12:43               ` Igor Ryzhov
2021-04-23 12:58                 ` Igor Ryzhov

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