[PATCH] remove TTY_DONT_FLIP

Remove TTY_DONT_FLIP tty flag.  This flag was introduced in 2.1.X kernels
to prevent the N_TTY line discipline functions read_chan() and
n_tty_receive_buf() from running at the same time.  2.2.15 introduced
tty->read_lock to protect access to the N_TTY read buffer, which is the
only state requiring protection between these two functions.

The current TTY_DONT_FLIP implementation is broken for SMP, and is not
universally honored by drivers that send data directly to the line
discipline receive_buf function.

Because TTY_DONT_FLIP is not necessary, is broken in implementation, and is
not universally honored, it is removed.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

4 files changed, 4 insertions, 20 deletions

diff --git a/drivers/char/mxser.c b/drivers/char/mxser.c
index 645d9d713aec..72cfd09091e0 100644
--- a/drivers/char/mxser.c
+++ b/drivers/char/mxser.c
@@ -996,7 +996,6 @@ static int mxser_open(struct tty_struct *tty, struct file *filp)
 
         info->session = current->signal->session;
         info->pgrp = process_group(current);
-        clear_bit(TTY_DONT_FLIP, &tty->flags);
 
         /*
         status = mxser_get_msr(info->base, 0, info->port);
diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index b9371d5bf790..603b9ade5eb0 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -1132,7 +1132,7 @@ static inline int input_available_p(struct tty_struct *tty, int amt)
  *      buffer, and once to drain the space from the (physical) beginning of
  *      the buffer to head pointer.
  *
- *      Called under the tty->atomic_read_lock sem and with TTY_DONT_FLIP set
+ *      Called under the tty->atomic_read_lock sem
  *
  */
  
@@ -1271,7 +1271,6 @@ do_it_again:
         }
 
         add_wait_queue(&tty->read_wait, &wait);
-        set_bit(TTY_DONT_FLIP, &tty->flags);
         while (nr) {
                 /* First test for status change. */
                 if (tty->packet && tty->link->ctrl_status) {
@@ -1315,9 +1314,7 @@ do_it_again:
                                 break;
                         }
                         n_tty_set_room(tty);
-                        clear_bit(TTY_DONT_FLIP, &tty->flags);
                         timeout = schedule_timeout(timeout);
-                        set_bit(TTY_DONT_FLIP, &tty->flags);
                         continue;
                 }
                 __set_current_state(TASK_RUNNING);
@@ -1394,7 +1391,6 @@ do_it_again:
                 if (time)
                         timeout = time;
         }
-        clear_bit(TTY_DONT_FLIP, &tty->flags);
         mutex_unlock(&tty->atomic_read_lock);
         remove_wait_queue(&tty->read_wait, &wait);
 
diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index 9b5a2c0e7008..0c17f61549b4 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -101,7 +101,7 @@ static void pty_unthrottle(struct tty_struct * tty)
  *
  * FIXME: Our pty_write method is called with our ldisc lock held but
  * not our partners. We can't just take the other one blindly without
- * risking deadlocks.  There is also the small matter of TTY_DONT_FLIP
+ * risking deadlocks.
  */
 static int pty_write(struct tty_struct * tty, const unsigned char *buf, int count)
 {
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index bd74e82d8a72..b846d87f2b56 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -784,11 +784,8 @@ restart:
         }
 
         clear_bit(TTY_LDISC, &tty->flags);
-        clear_bit(TTY_DONT_FLIP, &tty->flags);
-        if (o_tty) {
+        if (o_tty)
                 clear_bit(TTY_LDISC, &o_tty->flags);
-                clear_bit(TTY_DONT_FLIP, &o_tty->flags);
-        }
         spin_unlock_irqrestore(&tty_ldisc_lock, flags);
 
         /*
@@ -1955,7 +1952,6 @@ static void release_dev(struct file * filp)
          * race with the set_ldisc code path.
          */
         clear_bit(TTY_LDISC, &tty->flags);
-        clear_bit(TTY_DONT_FLIP, &tty->flags);
         cancel_delayed_work(&tty->buf.work);
 
         /*
@@ -2784,13 +2780,6 @@ static void flush_to_ldisc(void *private_)
         if (disc == NULL)       /*  !TTY_LDISC */
                 return;
 
-        if (test_bit(TTY_DONT_FLIP, &tty->flags)) {
-                /*
-                 * Do it after the next timer tick:
-                 */
-                schedule_delayed_work(&tty->buf.work, 1);
-                goto out;
-        }
         spin_lock_irqsave(&tty->buf.lock, flags);
         while((tbuf = tty->buf.head) != NULL) {
                 while ((count = tbuf->commit - tbuf->read) != 0) {
@@ -2809,7 +2798,7 @@ static void flush_to_ldisc(void *private_)
                 tty_buffer_free(tty, tbuf);
         }
         spin_unlock_irqrestore(&tty->buf.lock, flags);
-out:
+
         tty_ldisc_deref(disc);
 }