tty: Fix PPP hang under load

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

2 files changed, 22 insertions, 9 deletions

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index f307f135cbfb..7a84b406a952 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -316,8 +316,7 @@ struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
 {
         /* wait_event is a macro */
         wait_event(tty_ldisc_wait, tty_ldisc_try(tty));
-        if (tty->ldisc.refcount == 0)
-                printk(KERN_ERR "tty_ldisc_ref_wait\n");
+        WARN_ON(tty->ldisc.refcount == 0);
         return &tty->ldisc;
 }
 
@@ -376,15 +375,17 @@ EXPORT_SYMBOL_GPL(tty_ldisc_deref);
  *      @tty: terminal to activate ldisc on
  *
  *      Set the TTY_LDISC flag when the line discipline can be called
- *      again. Do necessary wakeups for existing sleepers.
+ *      again. Do necessary wakeups for existing sleepers. Clear the LDISC
+ *      changing flag to indicate any ldisc change is now over.
  *
- *      Note: nobody should set this bit except via this function. Clearing
- *      directly is allowed.
+ *      Note: nobody should set the TTY_LDISC bit except via this function.
+ *      Clearing directly is allowed.
  */
 
 void tty_ldisc_enable(struct tty_struct *tty)
 {
         set_bit(TTY_LDISC, &tty->flags);
+        clear_bit(TTY_LDISC_CHANGING, &tty->flags);
         wake_up(&tty_ldisc_wait);
 }
 
@@ -496,7 +497,14 @@ restart:
          *      reference to the line discipline. The TTY_LDISC bit
          *      prevents anyone taking a reference once it is clear.
          *      We need the lock to avoid racing reference takers.
+         *
+         *      We must clear the TTY_LDISC bit here to avoid a livelock
+         *      with a userspace app continually trying to use the tty in
+         *      parallel to the change and re-referencing the tty.
          */
+        clear_bit(TTY_LDISC, &tty->flags);
+        if (o_tty)
+                clear_bit(TTY_LDISC, &o_tty->flags);
 
         spin_lock_irqsave(&tty_ldisc_lock, flags);
         if (tty->ldisc.refcount || (o_tty && o_tty->ldisc.refcount)) {
@@ -528,7 +536,7 @@ restart:
          *      If the TTY_LDISC bit is set, then we are racing against
          *      another ldisc change
          */
-        if (!test_bit(TTY_LDISC, &tty->flags)) {
+        if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
                 struct tty_ldisc *ld;
                 spin_unlock_irqrestore(&tty_ldisc_lock, flags);
                 tty_ldisc_put(new_ldisc.ops);
@@ -536,10 +544,14 @@ restart:
                 tty_ldisc_deref(ld);
                 goto restart;
         }
-
-        clear_bit(TTY_LDISC, &tty->flags);
+        /*
+         *      This flag is used to avoid two parallel ldisc changes. Once
+         *      open and close are fine grained locked this may work better
+         *      as a mutex shared with the open/close/hup paths
+         */
+        set_bit(TTY_LDISC_CHANGING, &tty->flags);
         if (o_tty)
-                clear_bit(TTY_LDISC, &o_tty->flags);
+                set_bit(TTY_LDISC_CHANGING, &o_tty->flags);
         spin_unlock_irqrestore(&tty_ldisc_lock, flags);
         
         /*
diff --git a/include/linux/tty.h b/include/linux/tty.h
index f88169787a5f..bbbeaef99626 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -301,6 +301,7 @@ struct tty_struct {
 #define TTY_PUSH                6       /* n_tty private */
 #define TTY_CLOSING             7       /* ->close() in progress */
 #define TTY_LDISC               9       /* Line discipline attached */
+#define TTY_LDISC_CHANGING      10      /* Line discipline changing */
 #define TTY_HW_COOK_OUT         14      /* Hardware can do output cooking */
 #define TTY_HW_COOK_IN          15      /* Hardware can do input cooking */
 #define TTY_PTY_LOCK            16      /* pty private */