[PATCH] ugen detach race

[Anish Mistry]

	While working on getting hplip ported I ran across a race condition 
in the ugen code that causes a crash.  The following patch fixes a 
problem where read, write, and ioctl can be called during a detach 
since sc_dying isn't checked before bumping the reference count.  
This puts the sc_dying check before the *_do_* functions are called.  
This includes the patch from usb/81308 to prevent polling on the 
control endpoint.  As well as a few NULL pointer checks from NetBSD.  
This patch is applicable to RELENG_6.

http://am-productions.biz/docs/ugen-detach-race.patch

This doesn't fix the case where an application has a read/write 
pending and then detach is called.  In this case destroy_devl will 
just keep looping until the read/write completes.

-- 
Anish Mistry
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