PERFORCE change 146544 for review

[John Baldwin]

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).

-- 
John Baldwin