PERFORCE change 146544 for review

[Kostik Belousov]

On Thu, Aug 07, 2008 at 12:18:40PM -0400, John Baldwin wrote:
> On Sunday 03 August 2008 01:46:38 pm Robert Watson wrote:
> > 
> > On Sun, 3 Aug 2008, Ed Schouten wrote:
> > 
> > > http://perforce.freebsd.org/chv.cgi?CH=146544
> > >
> > > Change 146544 by ed at ed_flippo on 2008/08/03 17:36:53
> > >
> > > 	Add a counter to store the number of null modem devices.
> > 
> > I've seen similar constructs in lots of drivers, and it's basically an 
> unsafe 
> > construct because the deregister is non-atomic with respect to the busy 
> check. 
> > I wonder if, instead, we should use a mutex to protect the count, and set it 
> > to some value to deny future allocations, such as -1, once the ebusy check 
> is 
> > past...  Even that is unideal, though.
> 
> Yes, you basically need to set a flag of some sort to disable cloning, then go 
> tear down the clone handler.  And, yes, you probably should just use a mutex 
> for this (there are tricky games one could play with atomic ops, but given 
> how rare clone and detach happen, it isn't worth the code complexity).

I answered to Ed on the channel, the right incantation sequence is:
1. remove clone eventhandler;
2. call drain_dev_clone_events() to move past the point where new clones
   may be created;
3. do the destroy_dev() as needed;
4. if code ever called destroy_dev_sched(), call destroy_dev_drain(&csw).
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