RFC: possible issue with kqueue

[Ian Lepore]

On Tue, 2021-03-30 at 11:14 -0700, John-Mark Gurney wrote:
> Emanuel Haupt wrote this message on Sat, Mar 27, 2021 at 13:10 +0100:
> > Can someone familiar with kqueue please comment on:
> > 
> > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254024
> 
> Done:
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254024#c11
> 
> Looks like the user wasn't force unmounting the FS.  There really
> isn't any problem w/ kqueue, as a normal unmount is expected to be
> refused while files are open.
> 
> I guess there COULD be a new flag added to file descriptors that
> flag them as being able to be closed upon unmount.  Then when an
> unmount happens and only these flagged files remain, they are closed
> allowing the fs to unmount.  But this is a new feature and
> independent
> of kqueue.
> 

While it's not a kqueue bug per se, it is a weakness in freebsd that
there is no way to monitor a volume, or items within that volume,
without making it impossible to unmount the volume.  I've been fighting
this with various implementations of desktop software for like 20 years
on freebsd.  Usually I have to just disable all monitoring and live
with the reduced desktop functionality.

I wonder if a reasonable fix might be to have some sort of pre-unmount
event that can be delivered via kqueue, so that a userland entity
monitoring on that volume has a chance to close related descriptors so
that the unmount can proceed?

-- Ian