Re: [1/1] connector/CBUS: new messaging subsystem. Revision number next.

Evgeniy Polyakov
2005-05-10T18:26:07Z

On Tue, 10 May 2005 12:50:55 -0500
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

> On 5/10/05, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
> > On Tue, 10 May 2005 09:56:45 -0500
> > Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> >
> > > On 5/10/05, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
> > > > On Tue, 10 May 2005 01:18:46 -0500
> > > > Dmitry Torokhov <dtor_core@ameritech.net> wrote:
> > > >
> > > > > - drop cn_dev - there is only one connector;
> > > >
> > > > There may be several connectors with different socket types.
> > > > I use it in my environment for tests.
> > > >
> > >
> > > Why do you need that? u32 ID space is not enough?
> >
> > They use different netlink sockets.
> > One of them can fail during initialisation, for example...
>
> Are you saying that there more chances successfully allocating 2
> netlink sockets than one?

Of course - one may be in use by ipt_ULOG for example.
Although probably it could be better to get new unit from David Miller.
I want to create some kind of unused-in-kernel socket number -
each netlink user outside kernel might use it without kernel users
bothering.

> >
> > > > > - simplify cn_notify_request to carry only one range - it simplifies code
> > > > > quite a bit - nothing stops clientd from sending several notification
> > > > > requests;
> > > >
> > > > ? Nothing stops clients from sending several notification requests now.
> > > > I do not see how it simplifies the things?
> > > > Feel free to set idx_num and val_num to 1 and you will have the same.
> > >
> > > Compare the implementaion - without variable-length notification
> > > requests the code is much simplier and flexible.
> >
> > Hmm, old request had a length, new one - does not.
> > But instead of atomically addition of the whole set, one need to send
> > couple of events.
> > I do not see any profit here.
>
> Smaller kernel code, simpler notification structure, ability to stop
> listenig to a certain events without re-registering the rest of the
> block.

As I said - set idx and val numbers to one and you will have the same
behaviour.
Code is neither smaller [ok, you removed a loop] nor simplier
- it is functionality decreasing.

> > > > > - admit that SKBs are message medium and do not mess up with passing around
> > > > > destructor functions.
> > > >
> > > > You mess up layers.
> > > > I do not see why you want it.
> > >
> > > What layers? Connector core is not an onion, with my patch it is 481
> > > line .c file(down from 900+), comments probably take 1/3 of it. We
> > > just need to keep it simple, that's all.
> >
> > There is low-level medium layer - skbs.
> > There is middle-level layer - logical handling attached data and appropriate callback invocation.
> > There is high-level layer - various protocols over connector.
> >
> > Why any of them should know that others exist?
> > Your changes with destructor removing only saved < 10 lines of code, btw.
>
> Yes, line here and line there - all in all couple hundred lines.

It is not unused bits - it is framework which allows easy functionality extending.

> > > > Connector with your changes just does not work.
> > >
> > > As in "I tried to run it and adjusted the application for the new
> > > cn_notify_req size and it does not work" or "I am saying it does not
> > > work"?
> >
> > Second, i.e. "I am saing it does not work".
> > With your changes userspace just does not see any message being sent by kernelspace.
>
> Would you care to elaborate on this please. As far as I know
> kernel->userspace path was not changed at all.

I inserted your module and test module, which sends a message using cn_netlink_send()
one time per second.
Usersace application bound to -1 group does not receive that messages.
If I remove test module and your module, then insert vanilla
connector module and test module, then userspace application
begins to receive messages.
Here is part of dmesg:

cn_test_callback: 403117737: idx=123, val=457, seq=9, ack=0, len=12: counter = 9.
cn_test_callback: 403118737: idx=123, val=457, seq=10, ack=0, len=13: counter = 10.
cn_test_callback: 403119737: idx=123, val=457, seq=11, ack=0, len=13: counter = 11.
cn_test_callback: 403120737: idx=123, val=457, seq=12, ack=0, len=13: counter = 12.
connector: cn_cb_equal: comparing ffffffff.ffffffff and 0123.0456
connector: cn_cb_equal: comparing ffffffff.ffffffff and 0123.0457
connector: cn_cb_equal: comparing 0123.0456 and 0123.0457
connector: cn_cb_equal: comparing ffffffff.ffffffff and 0123.0457
connector: cn_cb_equal: comparing 0123.0456 and 0123.0457
connector: cn_cb_equal: comparing 0123.0457 and 0123.0457
connector: cn_netlink_send: len=12, seq=0, ack=0, group=0.

I can not say where the problem is and completely misunderstand how test callback
can be called several times...
Something is broken.

> >
> > > > Dmitry, I do not understand why do you change things in so radical manner,
> > > > especially when there are no 1. new interesting features, 2. no performance
> > > > improvements, 3. no right design changes.
> > >
> > > There is a right design change - it keeps the code simple. It peels
> > > all the layers you invented for something that does not really need
> > > layers - connector is just a simple wrapper over raw netlink.
> >
> > That is your opinion.
> >
> > Code is already as simple as possible, since it is layered and each
> > piece of code is separated from different logical entities.
> >
> > > When I look at the code I see the mess of unneeded refcounting, wrong
> > > kinds of locking, over-engineering. Of course I want to change it.
> >
> > There are no unneded reference counting now.
> > Locking was not wrong - you only placed semaphore instead of notify BH lock
> > just to have ability to sleep under the lock and send callback addition/removing
> > notification with GFP_KERNEL mask,
> > and it is probably right change from the first view, as I said,
> > but it can be implemented in a different way.
>
> That's what I was referring to as "wring locking". Notifications do
> not need to stop bottom handlers, a spinlock or semaphore willl work
> just fine. SInce I wanted to allocate memory only after I determined
> that it is a new notify request a semaphore was a better choise.

I will think of it, but in long-time I do not like such approach -
it is some kind of deadlock - we have no memory, so we sleep in allocation,
so we can not remove callback since it will sleep in allocation...

> > > Keep it simple. Except for your testing environment why do you need to
> > > have separate netlink sockets? You have an ample ID space. Why do you
> > > need separate cn_dev and cn_queue_dev (and in the end have them at
> > > all)? Why do you need "input" handler, do you expect to use different
> > > handlers? If so which? And why? Do you expect to use other transports?
> > > If so which? And why? You need to have justifiction for every
> > > additional layer you putting on.
> >
> > Sigh.
> > It was already answered so many times, as far as I remember...
> > Ok, let's do it again.
> >
> > Several netlink sockets - if you can see, connector uses NETLINK_ULOG
> > for it's notification, you can not use the same socket twice, so you
> > may have new device with new socket, or several of them, and system
> > may work if one initialisation failed.
>
> Or it may not. Why do you think that having several sockets is safer
> than using one? After all, it is just a matter of setting it up and
> finally allocating a separate socket number for connector.

Yes, there can be such a way.
But when connector was not in mainline it used TUN/TAP netlink sockets
and tried to get one by one, until successfully allocated.
Using dmesg userspace could find needed socket number for itself.

> > cn_dev is connector device structure, cn_queue_dev - it's queueing part.
> > input handler is needed for ability to receive messages.
>
> You chose not to understand my question. Why do you need to have is as
> a pointer? Do you expect to have different input handler?

In runtime - no.
cn_dev is a main transport structure, while cn_queue_dev is for
callback queue handling.

> > Couple of next questions can be answered in one reply:
> > original connector's idea was to implement notification based
> > not on top of netlink protocol, but using ioctls of char device,
> > so there could be different connector device, input method,
> > transport and so on. Probably I will not implement it thought.
>
> IOW this all is remnants of an older design. You have no plans to
> utilize the framework and therefore it is not needed anymore and can
> be dropped.

Different connector devices still there.
They use different sockets.
Concider netlink2 [with guaranteed delivering - I have some ideas about it] -
one just needs to add a new connector device, no new braindumping and
recalling how it was cool before.
It will require some changes to the connector structure, but it will not
be complete rewrite because some time ago someone decided we do not
need it and completely removed anything that lives outside simple
netlink wrapper.

> --
> Dmitry

Evgeniy Polyakov

Only failure makes us experts. -- Theo de Raadt