tty: rewrite the ldisc locking

There are several pretty much unfixable races in the old ldisc code, especially
with respect to pty behaviour and also to hangup. It's easier to rewrite the
code than simply try and patch it up.

This patch
- splits the ldisc from the tty (so we will be able to refcount it more cleanly
  later)
- introduces a mutex lock for ldisc changing on an active device
- fixes the complete mess that hangup caused
- implements hopefully correct setldisc/close/hangup locking

There are still some problems around pty pairs that have always been there but
at least it is now possible to understand the code and fix further problems.

This fixes the following known bugs
- hang up can leak ldisc references
- hang up may not call open/close on ldisc in a matched way
- pty/tty pairs can deadlock during an ldisc change
- reading the ldisc proc files can cause every ldisc to be loaded

and probably a few other of the mysterious ldisc race reports.

I'm sure it also adds the odd new one.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 56 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index be49d0730bb9..2f44b0b241c3 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -492,22 +492,6 @@ void tty_ldisc_flush(struct tty_struct *tty)
 EXPORT_SYMBOL_GPL(tty_ldisc_flush);
 
 /**
- *      tty_reset_termios       -       reset terminal state
- *      @tty: tty to reset
- *
- *      Restore a terminal to the driver default state
- */
-
-static void tty_reset_termios(struct tty_struct *tty)
-{
-        mutex_lock(&tty->termios_mutex);
-        *tty->termios = tty->driver->init_termios;
-        tty->termios->c_ispeed = tty_termios_input_baud_rate(tty->termios);
-        tty->termios->c_ospeed = tty_termios_baud_rate(tty->termios);
-        mutex_unlock(&tty->termios_mutex);
-}
-
-/**
  *      do_tty_hangup           -       actual handler for hangup events
  *      @work: tty device
  *
@@ -536,7 +520,6 @@ static void do_tty_hangup(struct work_struct *work)
         struct file *cons_filp = NULL;
         struct file *filp, *f = NULL;
         struct task_struct *p;
-        struct tty_ldisc *ld;
         int    closecount = 0, n;
         unsigned long flags;
         int refs = 0;
@@ -567,40 +550,8 @@ static void do_tty_hangup(struct work_struct *work)
                 filp->f_op = &hung_up_tty_fops;
         }
         file_list_unlock();
-        /*
-         * FIXME! What are the locking issues here? This may me overdoing
-         * things... This question is especially important now that we've
-         * removed the irqlock.
-         */
-        ld = tty_ldisc_ref(tty);
-        if (ld != NULL) {
-                /* We may have no line discipline at this point */
-                if (ld->ops->flush_buffer)
-                        ld->ops->flush_buffer(tty);
-                tty_driver_flush_buffer(tty);
-                if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
-                    ld->ops->write_wakeup)
-                        ld->ops->write_wakeup(tty);
-                if (ld->ops->hangup)
-                        ld->ops->hangup(tty);
-        }
-        /*
-         * FIXME: Once we trust the LDISC code better we can wait here for
-         * ldisc completion and fix the driver call race
-         */
-        wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
-        wake_up_interruptible_poll(&tty->read_wait, POLLIN);
-        /*
-         * Shutdown the current line discipline, and reset it to
-         * N_TTY.
-         */
-        if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
-                tty_reset_termios(tty);
-        /* Defer ldisc switch */
-        /* tty_deferred_ldisc_switch(N_TTY);
 
-          This should get done automatically when the port closes and
-          tty_release is called */
+        tty_ldisc_hangup(tty);
 
         read_lock(&tasklist_lock);
         if (tty->session) {
@@ -629,12 +580,15 @@ static void do_tty_hangup(struct work_struct *work)
         read_unlock(&tasklist_lock);
 
         spin_lock_irqsave(&tty->ctrl_lock, flags);
-        tty->flags = 0;
+        clear_bit(TTY_THROTTLED, &tty->flags);
+        clear_bit(TTY_PUSH, &tty->flags);
+        clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
         put_pid(tty->session);
         put_pid(tty->pgrp);
         tty->session = NULL;
         tty->pgrp = NULL;
         tty->ctrl_status = 0;
+        set_bit(TTY_HUPPED, &tty->flags);
         spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 
         /* Account for the p->signal references we killed */
@@ -660,10 +614,7 @@ static void do_tty_hangup(struct work_struct *work)
          * can't yet guarantee all that.
          */
         set_bit(TTY_HUPPED, &tty->flags);
-        if (ld) {
-                tty_ldisc_enable(tty);
-                tty_ldisc_deref(ld);
-        }
+        tty_ldisc_enable(tty);
         unlock_kernel();
         if (f)
                 fput(f);
@@ -2570,7 +2521,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
         case TIOCGSID:
                 return tiocgsid(tty, real_tty, p);
         case TIOCGETD:
-                return put_user(tty->ldisc.ops->num, (int __user *)p);
+                return put_user(tty->ldisc->ops->num, (int __user *)p);
         case TIOCSETD:
                 return tiocsetd(tty, p);
         /*
@@ -2785,6 +2736,7 @@ void initialize_tty_struct(struct tty_struct *tty,
         tty->buf.head = tty->buf.tail = NULL;
         tty_buffer_init(tty);
         mutex_init(&tty->termios_mutex);
+        mutex_init(&tty->ldisc_mutex);
         init_waitqueue_head(&tty->write_wait);
         init_waitqueue_head(&tty->read_wait);
         INIT_WORK(&tty->hangup_work, do_tty_hangup);