Re: Grace period

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10.04.2012 03:26, bfields@xxxxxxxxxxxx пишет:
On Mon, Apr 09, 2012 at 03:24:19PM +0400, Stanislav Kinsbursky wrote:
07.04.2012 03:40, bfields@xxxxxxxxxxxx пишет:
On Fri, Apr 06, 2012 at 09:08:26PM +0400, Stanislav Kinsbursky wrote:
Hello, Bruce.
Could you, please, clarify this reason why grace list is used?
I.e. why list is used instead of some atomic variable, for example?

Like just a reference count?  Yeah, that would be OK.

In theory it could provide some sort of debugging help.  (E.g. we could
print out the list of "lock managers" currently keeping us in grace.)  I
had some idea we'd make those lock manager objects more complicated, and
might have more for individual containerized services.

Could you share this idea, please?

Anyway, I have nothing against lists. Just was curious, why it was used.
I added Trond and lists to this reply.

Let me explain, what is the problem with grace period I'm facing
right know, and what I'm thinking about it.
So, one of the things to be containerized during "NFSd per net ns"
work is the grace period, and these are the basic components of it:
1) Grace period start.
2) Grace period end.
3) Grace period check.
3) Grace period restart.

For restart, you're thinking of the fs/lockd/svc.c:restart_grace()
that's called on aisngal in lockd()?

I wonder if there's any way to figure out if that's actually used by
anyone?  (E.g. by any distro init scripts).  It strikes me as possibly
impossible to use correctly.  Perhaps we could deprecate it....


Or (since lockd kthread is visible only from initial pid namespace) we can just hardcode "init_net" in this case. But it means, that this "kill" logic will be broken if two containers shares one pid namespace, but have separated networks namespaces.
Anyway, both (this one or Bruce's) solutions suits me.

So, the simplest straight-forward way is to make all internal stuff:
"grace_list", "grace_lock", "grace_period_end" work and both
"lockd_manager" and "nfsd4_manager" - per network namespace. Also,
"laundromat_work" have to be per-net as well.
In this case:
1) Start - grace period can be started per net ns in
"lockd_up_net()" (thus has to be moves there from "lockd()") and
"nfs4_state_start()".
2) End - grace period can be ended per net ns in "lockd_down_net()"
(thus has to be moved there from "lockd()"), "nfsd4_end_grace()" and
"fs4_state_shutdown()".
3) Check - looks easy. There is either svc_rqst or net context can
be passed to function.
4) Restart - this is a tricky place. It would be great to restart
grace period only for the networks namespace of the sender of the
kill signal. So, the idea is to check siginfo_t for the pid of
sender, then try to locate the task, and if found, then get sender's
networks namespace, and restart grace period only for this namespace
(of course, if lockd was started for this namespace - see below).

If it's really the signalling that's the problem--perhaps we can get
away from the signal-based interface.

At least in the case of lockd I suspect we could.


I'm ok with that. So, if no objections will follow, I'll drop it and send the patch. Or you want to do it?

BTW, I tried this "pid from siginfo" approach yesterday. And it doesn't work, because sender usually dead already, when lookup for task by pid is performed.

Or perhaps the decision to share a single lockd thread (or set of nsfd
threads) among multiple network namespaces was a poor one.  But I
realize multithreading lockd doesn't look easy.


This decision was the best one in current circumstances.
Having Lockd thread (or NFSd threads) per container looks easy to implement on first sight. But kernel threads currently supported only in initial pid namespace. I.e. it means that per-container kernel thread won't be visible in container, if it has it's own pid namespace. And there is no way to put a kernel thread into container. In OpenVZ we have per-container kernel threads. But integrating this feature to mainline looks hopeless (or very difficult) to me. At least for now.
So this problem with signals remains unsolved.

So, as it looks to me, this "one service per all" is the only one suitable for now. But there are some corner cases which have to be solved.

Anyway, Jeff's question is still open.
Do we need to prevent people from exporting nested directories from different network namespaces?
And if yes, how to do this?

--
Best regards,
Stanislav Kinsbursky
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