tty: Remove tty_hung_up_p() tests from tty drivers' open()
authorPeter Hurley <peter@hurleysoftware.com>
Mon, 16 Jun 2014 13:17:06 +0000 (09:17 -0400)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 10 Jul 2014 23:06:49 +0000 (16:06 -0700)
commite359a4e38d229d53e28905863a1fabf41debd591
treec70f7762dac31dad62188b7b8bb2140bea8b311a
parent5fda7a0e715c32c6dd9a39ebce8429eeafb64b7d
tty: Remove tty_hung_up_p() tests from tty drivers' open()

Since at least before 2.6.30, it has not been possible to observe
a hung up file pointer in a tty driver's open() method unless/until
the driver open() releases the tty_lock() (eg., before blocking).

This is because tty_open() adds the file pointer while holding
the tty_lock() _and_ doesn't release the lock until after calling
the tty driver's open() method. [ Before tty_lock(), this was
lock_kernel(). ]

Since __tty_hangup() first waits on the tty_lock() before
enumerating and hanging up the open file pointers, either
__tty_hangup() will wait for the tty_lock() or tty_open() will
not yet have added the file pointer. For example,

CPU 0                          |  CPU 1
                               |
tty_open                       |  __tty_hangup
  ..                           |    ..
  tty_lock                     |    ..
  tty_reopen                   |    tty_lock  / blocks
  ..                           |
  tty_add_file(tty, filp)      |
  ..                           |
  tty->ops->open(tty, filp)    |
    tty_port_open              |
      tty_port_block_til_ready |
        ..                     |
        while (1)              |
          ..                   |
          tty_unlock           |    / unblocks
          schedule             |    for each filp on tty->tty_files
                               |      f_ops = tty_hung_up_fops;
                               |    ..
                               |    tty_unlock
          tty_lock             |
  ..                           |
  tty_unlock                   |

Note that since tty_port_block_til_ready() and similar drop
the tty_lock while blocking, when woken, the file pointer
must then be tested for having been hung up.

Also, fix bit-rotted drivers that used extra_count to track the
port->count bump.

CC: Mikael Starvik <starvik@axis.com>
CC: Samuel Ortiz <samuel@sortiz.org>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/char/pcmcia/synclink_cs.c
drivers/staging/dgrp/dgrp_tty.c
drivers/tty/cyclades.c
drivers/tty/serial/crisv10.c
drivers/tty/serial/serial_core.c
drivers/tty/synclink.c
drivers/tty/synclink_gt.c
drivers/tty/synclinkmp.c
drivers/tty/tty_port.c
net/irda/ircomm/ircomm_tty.c