No need to take the global tty_mutex, signal->tty->driver can't go away while
we are holding ->siglock.
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>