tty: Prepare for destroying line discipline on hangup
authorPeter Hurley <peter@hurleysoftware.com>
Mon, 11 Jan 2016 06:41:01 +0000 (22:41 -0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 27 Jan 2016 23:01:44 +0000 (15:01 -0800)
commite55afd11a48354c810caf6b6ad4c103016a88230
treeeb073c28c1e21785fcbe92740ce1290da7fcb85c
parent5b6e6832f41c9422753804671ee06af37ac8caf6
tty: Prepare for destroying line discipline on hangup

tty file_operations (read/write/ioctl) wait for the ldisc reference
indefinitely (until ldisc lifetime events, such as hangup or TIOCSETD,
finish). Since hangup now destroys the ldisc and does not instance
another copy, file_operations must now be prepared to receive a NULL
ldisc reference from tty_ldisc_ref_wait():

CPU 0                                   CPU 1
-----                                   -----
(*f_op->read)() => tty_read()
                                        __tty_hangup()
                                        ...
                                        f_op = &hung_up_tty_fops;
                                        ...
                                        tty_ldisc_hangup()
                                           tty_ldisc_lock()
                                           tty_ldisc_kill()
                                              tty->ldisc = NULL
                                           tty_ldisc_unlock()
ld = tty_ldisc_ref_wait()
/* ld == NULL */

Instead, the action taken now is to return the same value as if the
tty had been hungup a moment earlier:

CPU 0                                   CPU 1
-----                                   -----
                                        __tty_hangup()
                                        ...
                                        f_op = &hung_up_tty_fops;
(*f_op->read)() => hung_up_tty_read()
return 0;
                                        ...
                                        tty_ldisc_hangup()
                                           tty_ldisc_lock()
                                           tty_ldisc_kill()
                                              tty->ldisc = NULL
                                           tty_ldisc_unlock()

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/tty/tty_io.c
drivers/tty/tty_ldisc.c
drivers/tty/vt/selection.c