releases/2.6.36.2/tty-restore-tty_ldisc_wait_idle.patch

   1 From 100eeae2c5ce23b4db93ff320ee330ef1d740151 Mon Sep 17 00:00:00 2001
   2 From: Jiri Slaby <jslaby@suse.cz>
   3 Date: Sun, 31 Oct 2010 23:17:51 +0100
   4 Subject: TTY: restore tty_ldisc_wait_idle
   5
   6 From: Jiri Slaby <jslaby@suse.cz>
   7
   8 commit 100eeae2c5ce23b4db93ff320ee330ef1d740151 upstream.
   9
  10 It was removed in 65b770468e98 (tty-ldisc: turn ldisc user count into
  11 a proper refcount), but we need to wait for last user to quit the
  12 ldisc before we close it in tty_set_ldisc.
  13
  14 Otherwise weird things start to happen. There might be processes
  15 waiting in tty_read->n_tty_read on tty->read_wait for input to appear
  16 and at that moment, a change of ldisc is fatal. n_tty_close is called,
  17 it frees read_buf and the waiting process is still in the middle of
  18 reading and goes nuts after it is woken.
  19
  20 Previously we prevented close to happen when others are in ldisc ops
  21 by tty_ldisc_wait_idle in tty_set_ldisc. But the commit above removed
  22 that. So revoke the change and test whether there is 1 user (=we), and
  23 allow the close then.
  24
  25 We can do that without ldisc/tty locks, because nobody else can open
  26 the device due to TTY_LDISC_CHANGING bit set, so we in fact wait for
  27 everybody to leave.
  28
  29 I don't understand why tty_ldisc_lock would be needed either when the
  30 counter is an atomic variable, so this is a lockless
  31 tty_ldisc_wait_idle.
  32
  33 On the other hand, if we fail to wait (timeout or signal), we have to
  34 reenable the halted ldiscs, so we take ldisc lock and reuse the setup
  35 path at the end of tty_set_ldisc.
  36
  37 Signed-off-by: Jiri Slaby <jslaby@suse.cz>
  38 Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
  39 Tested-by: Sebastian Andrzej Siewior <bigeasy@breakpoint.cc>
  40 LKML-Reference: <20101031104136.GA511@Chamillionaire.breakpoint.cc>
  41 LKML-Reference: <1287669539-22644-1-git-send-email-jslaby@suse.cz>
  42 Cc: Alan Cox <alan@linux.intel.com>
  43 Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
  44
  45 ---
  46  drivers/char/tty_ldisc.c |   29 +++++++++++++++++++++++++++++
  47  1 file changed, 29 insertions(+)
  48
  49 --- a/drivers/char/tty_ldisc.c
  50 +++ b/drivers/char/tty_ldisc.c
  51 @@ -47,6 +47,7 @@
  52
  53  static DEFINE_SPINLOCK(tty_ldisc_lock);
  54  static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
  55 +static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_idle);
  56  /* Line disc dispatch table */
  57  static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
  58
  59 @@ -83,6 +84,7 @@ static void put_ldisc(struct tty_ldisc *
  60                 return;
  61         }
  62         local_irq_restore(flags);
  63 +       wake_up(&tty_ldisc_idle);
  64  }
  65
  66  /**
  67 @@ -531,6 +533,23 @@ static int tty_ldisc_halt(struct tty_str
  68  }
  69
  70  /**
  71 + *     tty_ldisc_wait_idle     -       wait for the ldisc to become idle
  72 + *     @tty: tty to wait for
  73 + *
  74 + *     Wait for the line discipline to become idle. The discipline must
  75 + *     have been halted for this to guarantee it remains idle.
  76 + */
  77 +static int tty_ldisc_wait_idle(struct tty_struct *tty)
  78 +{
  79 +       int ret;
  80 +       ret = wait_event_interruptible_timeout(tty_ldisc_idle,
  81 +                       atomic_read(&tty->ldisc->users) == 1, 5 * HZ);
  82 +       if (ret < 0)
  83 +               return ret;
  84 +       return ret > 0 ? 0 : -EBUSY;
  85 +}
  86 +
  87 +/**
  88   *     tty_set_ldisc           -       set line discipline
  89   *     @tty: the terminal to set
  90   *     @ldisc: the line discipline
  91 @@ -634,8 +653,17 @@ int tty_set_ldisc(struct tty_struct *tty
  92
  93         flush_scheduled_work();
  94
  95 +       retval = tty_ldisc_wait_idle(tty);
  96 +
  97         tty_lock();
  98         mutex_lock(&tty->ldisc_mutex);
  99 +
 100 +       /* handle wait idle failure locked */
 101 +       if (retval) {
 102 +               tty_ldisc_put(new_ldisc);
 103 +               goto enable;
 104 +       }
 105 +
 106         if (test_bit(TTY_HUPPED, &tty->flags)) {
 107                 /* We were raced by the hangup method. It will have stomped
 108                    the ldisc data and closed the ldisc down */
 109 @@ -669,6 +697,7 @@ int tty_set_ldisc(struct tty_struct *tty
 110
 111         tty_ldisc_put(o_ldisc);
 112
 113 +enable:
 114         /*
 115          *      Allow ldisc referencing to occur again
 116          */