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Re: the virtual device management in eth-multiplexer

From: olafBuddenhagen
Subject: Re: the virtual device management in eth-multiplexer
Date: Sun, 8 Feb 2009 21:48:47 +0100
User-agent: Mutt/1.5.18 (2008-05-17)


On Sun, Feb 01, 2009 at 02:20:41AM +0000, Da Zheng wrote:

> In the  current implementation, the device and the node associated to
> it are  created when file_name_lookup() is called. when the last
> client closes  the virtual device, the device is destroyed along with
> the node.
> However, there are several problems: If the program that access the
> device file is "ls", the behavior is  quite strange. if "ls" accesses
> a device file, for example "ls veth0" is  called, eth-multiplexer
> creates the node and the virtual device. since  "ls" never open the
> device, the device is never destroyed and the device  file always
> exists in the directory.

This is a bug. The node should go away when it no longer has any users.
When pfinet (or some other real user) uses a device, it will probably
discard the port to the FS node (resulting from file_name_lookup() )
after the device_open(), but it still has the port for the actual
device, so the node is in use -- the open device keeps the node around.
When the client closes the device, there are no references anymore, and
the node is destroyed.

ls on the other hand never does device_open(). The moment it discards
the port from file_name_lookup(), the node isn't referenced anymore, and
should be destroyed immediately, without the device ever having been

> I can postpone the creation of the virtual device. for example, it can
> be created when the client calls device_open. so, when "ls" is called
> with a device name, only the node is created.

Well, creating the device just means creating some internal data
structures, right? So doing it at the time of node creation, or only for
the time where the device is actually open, seems an implementation
detail really... Having the distinction between nodes and devices more
explicit might perhaps make the implementation clearer; I can't tell.

The only difference it would make to the outside world is that if
someone still holds a port to the node, while the actual device is
closed, any status data would be kept, and would be preserved when
someone opens the device again before the port is dropped. I'm not sure
whether this case is very relevant, and what behaviour would be more

> The next time when "ls" is  called on the directory, "ls" doesn't
> display the device. This behavior  is still as strange as the one
> above.

Well, it's certainly a bit unusual that you can ls an arbitrary name and
it will work, but the name doesn't show up in a directory listing... But
I don't think it will cause any serious problems. It's certainly *not*
as strange as the present (buggy) behaviour.

> Another problem is that if the device is created in
> file_name_lookup(),  any user can create a device as long as the user
> has the read permission  of the directory where eth-multiplexer sits.

Well, the user can open an arbitrary dynamically-created device for
reading. I'm don't know whether this makes any sense for network
devices; but it doesn't really seem wrong...

> so maybe it is better to give up the idea that the directory where
> eth-multiplexer sits is the place to show the status of devices and
> allow the client to operate the device file just like the normal file.
> for example, before accessing the device, the client has to create a
> device with "touch eth0" and afterwards, destroy it with "rm eth0".

I already considered such a possibility: allow explicitely creating
static devices *in addition* to the dynamic ones... Though I wonder
whether in this case it wouldn't be better simply to set up devnode
instances explicitely.

In any case, I want to keep the dynamic nodes -- it's just much more

Without dynamic devices, we would need a way to store the device
configuration permanently, so that we don't need to explicitely set it
up before launching pfinet. This is possible, but I'm not sure whether
it is really desirable...

I'm more and more inclined towards factoring the node management out of
eth-multiplexer, and handling it by some external means: This would give
us much more flexibility here.


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