TTY: ldisc, wait for idle ldisc in release
TTY: ldisc, wait for idle ldisc in release

We reintroduced tty_ldisc_wait_idle in 100eeae2c5c (TTY: restore
tty_ldisc_wait_idle) and used in set_ldisc. Then we added it also to
the hangup path in 92f6fa09bd453 (TTY: ldisc, do not close until there
are readers). And we noted that there is one more path:
~   Before 65b770468e98 tty_ldisc_wait_idle was called also from
~   tty_ldisc_release. It is called from tty_release, so I don't think
~   we need to restore that one.

Well, I was wrong. There might still be holders of an ldisc
reference. Not from userspace, but drivers. If they take a reference
and a user closes the device immediately after that, we have a
problem. ldisc is halted and closed by TTY, but the driver still may
call some ldisc's operation and cause a crash.

So restore the tty_ldisc_wait_idle call also to the third location
where it was before 65b770468e98 (tty-ldisc: turn ldisc user count
into a proper refcount). Now we should be safe with respect to the
ldisc reference counting as all* tty_ldisc_close paths are safely
called with reference count of one.

* Not the one in tty_ldisc_setup's fail path. But that is called
  before the first open finishes. So userspace does not see it yet.
  Even thought the driver is given the TTY already via ->install, it
  should not take a reference to the ldisc yet. If some driver is to
  do this, we should put one tty_ldisc_wait_idle also in the setup.

Signed-off-by: Jiri Slaby <dfbf39eb5bf303a3db5454d815cb8c8888cbadf9@suse.cz>
Acked-by: Alan Cox <91e38e63b890fbb214c8914809fde03c73e7f24d@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@linuxfoundation.org>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs