[patch] turning devctl into a "multiple openable" device

[Kostik Belousov]

On Wed, Nov 30, 2011 at 10:05:11AM -0500, John Baldwin wrote:
> On Wednesday, November 30, 2011 7:43:20 am Baptiste Daroussin wrote:
> > Hi all,
> > 
> > With the help of cognet, I wrote a patch to turn devctl into a multiple openable
> > device, that mean that it will allow to open /dev/devctl in multiple programs,
> > for example hald and everythings that want to receive notification from the
> > device won't need to depend on haveing devd running.
> > 
> > here is the patch: 
> > http://people.freebsd.org/~bapt/devctl_multi_open.diff
> 
> Shouldn't devctl_queue_data_f() use the requested malloc() flags instead of
> hardcoding M_NOWAIT?
This is an obvious fallback of holding mutex around the call to
per_devctl_queue_data_f(), which caused the author a trouble to use
M_WAITOK.

Having n readers causes the patch to queue each message n times, that looks
like a waste.

I wonder why the waiting_threads stuff is needed at all. The cv could
be woken up unconditionally everytime. What is the reason for the cv_wait
call in cdevpriv data destructor ? You cannot have a thread doing e.g.
read on the file descriptor while destructor is run.

> 
> Also, I know that it was an intentional design decisison by Warner to have
> the multiplexing of devctl data done in userland via devd rather than in the
> kernel.  (I think he envisioned devd providing a UNIX domain socket or some
> such for other daemons to use to listen to events.)  Have you asked him about
> this change?
And I fully agree that doing multiplexing in user mode is the right approach.
Not least because you could apply some advanced access control and provide
filtering for the consumers.
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