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

On Wed, Nov 30, 2011 at 04:55:21PM +0100, Olivier Houchard wrote:
> On Wed, Nov 30, 2011 at 05:46:36PM +0200, Kostik Belousov wrote:
> > 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.
> > 
> 
> Queuing the message only one time would require to somehow keep a state, to
> know which thread read which message, and figuring out when to free a message
> can be an headache. Given I don't think they'll be a lot of readers, I'm not
> sure it's worth the trouble.
> 
> > 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.
> > 
> 
> What will prevent you from having a thread stuck in read(), while an another 
> one close() the fd ?
> 
Nothing, but file reference count goes to zero only after the thread
stuck in read is unstuck. Cdevpriv destructor is run only when file
reference count becomes zero, i.e. there can be no any accessing threads,
and new accesses are impossible since file descriptors also own references
on the file.