vgone() calling VOP_CLOSE() -> blocked threads?

[Bruce Evans]

On Sat, 15 Mar 2008, Kostik Belousov wrote:

> On Sun, Mar 16, 2008 at 03:55:18AM +1100, Bruce Evans wrote:
>> Other problems near here:
>> - neither vfs nor drivers currently know how many threads are in a
>>   driver.  vfs uses vp->v_rdev->si_usecount, but this doesn't quite work
> This is provided by si_threadcount.
> See the dev(vn)_refthread and it usage in the devfs vnops and fops.

So why doesn't reovoke() use it? :-).  All uses of si_usecount, which
normally happen via vcount() and count_dev(), are suspect, especially
the latter.

vcount() is only used in revoke(), in svr4_fcntl.c to handle another
revoke(), and for FreeBSD < 6 in reiserfs for an old multiple-mount
check.

count_dev() is only used in ata-tape.c (to decide in the same broken
way as vfs if a close is the last one -- this driver uses D_TRACKCLOSE
to get d_close() called on all closes.  This gives it the burden of
deciding whether the close is the last one, and it can't do this any
better than vfs.  D_TRACKCLOSE is used in a few other drivers which
don't call count_dev()), in devfs_close() (to decide whether to release
the controlling terminal and to decide when to call d_close()).

Hmm, it seems to be not vfs but only devfs which handles last-close
specially.  devfs is closer to devices, so it should know how to use
si_threadcount here.  Hopefully si_threadcount counts threads sleeping
in open or close, although si_usecount doesn't.  d_close (or something)
should be called to wake up these threads even if si_usecount is 0.
Drivers which support sleeping in open or close must support d_close
(or something) being called to forcibly end such sleeps.  revoke()
should forcibly end such sleeps, so it needs to check si_threadcount
too.  si_usecount in its current form might end up being unused, so
si_threadcount could be renamed back to it.

Bruce