tty locking again :)

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi Alan !

(Asking you since I know you did some TTY locking work a while ago).

I noticed that there doesn't seem to be any kind of locking in
tty_(un)register_driver. It can very easily race with tty_open() doing a
get_tty_driver(). Shouldn't tty_(un)register_driver be changed to take
the tty_sem at least while manipulating the list ?

I noticed that while chasing a different bug (a driver bug actually),
but I don't see how we are protected here. And considering the race I
found in the driver, I tend to think we aren't protected at all

FYI. The hvc driver issue was basically that it was allowing struct
console->device (the kernel console callback to link to the tty driver)
to return the tty_driver pointer before it called tty_register_driver,
thus a racing tty_open() of the default console was blow up accessing
driver->ttys.

Ben.