linux-kernel - Re: [PATCH v3] SUNRPC: protect service sockets lists during per-net shutdown

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Message-ID: <503354A0.5010707@parallels.com>
Date:	Tue, 21 Aug 2012 13:28:00 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>,
	"neilb@...e.de" <neilb@...e.de>
Subject: Re: [PATCH v3] SUNRPC: protect service sockets lists during per-net
 shutdown

20.08.2012 20:58, J. Bruce Fields пишет:
> On Mon, Aug 20, 2012 at 07:11:00PM +0400, Stanislav Kinsbursky wrote:
>> 20.08.2012 18:56, J. Bruce Fields пишет:
>>> On Mon, Aug 20, 2012 at 03:05:49PM +0400, Stanislav Kinsbursky wrote:
>>>> 16.08.2012 23:29, J. Bruce Fields пишет:
>>>>> Looking back at this:
>>>>>
>>>>> 	- adding the sv_lock looks like the right thing to do anyway
>>>>> 	  independent of containers, because svc_age_temp_xprts may
>>>>> 	  still be running.
>>>>>
>>>>> 	- I'm increasingly unhappy about sharing rpc servers between
>>>>> 	  network namespaces.  Everything would be easier to understand
>>>>> 	  if they were independent.  Can we figure out how to do that?
>>>>>
>>>>
>>>> Could you, please, elaborate on your your unhappiness?
>>>
>>> It seems like you're having to do a lot of work on each individual rpc
>>> server (callback server, lockd, etc.) to make per-net startup/shutdown
>>> work.  And then we still don't have it quite right (see the shutdown
>>> races).)
>>>
>>> In general whenever we have the opportunity to have entirely separate
>>> data structures, I'd expect that to simplify things: it should eliminate
>>> some locking and reference-counting issues.
>>>
>>
>> Agreed. But current solution still looks like the easies way to me
>> to implement desired functionality.
>>
>>>> I.e. I don't like it too. But the problem here, is that rpc server
>>>> is tied with kernel threads creation and destruction. And these
>>>> threads can be only a part of initial pid namespace (because we have
>>>> only one kthreadd). And we decided do not create new kernel thread
>>>> per container when were discussing the problem last time.
>>>
>>> There really should be some way to create a kernel thread in a specific
>>> namespace, shouldn't there?
>>>
>>
>>
>> Kthreads support in a container is rather a "political" problem,
>> than an implementation problem.
>
> Is there a mail thread somewhere with a summary of the objections?
>

I can't specify right now. Need to search over lkml history.
That's all what I've found for now:
http://us.generation-nt.com/patch-cgroups-disallow-attaching-kthreadd-help-207003852.html

>> Currently, when you call kthread_create(), you add new job to
>> kthreadd queue. Kthreadd is unique, starts right after init and
>> lives in global initial environment. So, any kthread inherits
>> namespaces from it.
>> Of course, we can start one kthread per environment and change it's
>> root or even network namespace in kthread function. But pid
>> namespace of this kthread will remain global.
>
> OK.  But the current implementation will leave all the server threads in
> the initial pid namespace, too.
>
>> It looks like not a big problem, when we shutdown kthread by some
>> variable. But what about killable nfsd kthreads?
>
> And we're stuck with that problem either way too, aren't we?
>

Yes, we are. But at least we are avoiding patching of task subsystem.

>> 1) We can't kill them from nested pid namespace.
>> 2) How we will differ nfsd kthreads in initial pid namespace?
>
> I have to admit for my purposes I don't care too much about pid
> namespaces or about signalling server threads.  It'd be nice to get
> those things right but it wouldn't bother me that much not to.
>
> Another stupid idea: can we do our own implementation of something like
> kthreadd just for the purpose of starting rpc server threads?  It
> doesn't seem that complicated.
>

Gm...
This idea is not stupid. If I understand you right, you suggest to implement a 
service per network namespace (i.e. not only data, but also threads)?

> --b.
>
>> In OpenVZ we have kthreadd per pid hamespace and it allows us to
>> create kthreads (and thus services) per pid namespace.


-- 
Best regards,
Stanislav Kinsbursky
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