tty: Add lock/unlock ldisc pair functions

Just as the tty pair must be locked in a stable sequence
(ie, independent of which is consider the 'other' tty), so must
the ldisc pair be locked in a stable sequence as well.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 87 insertions, 0 deletions

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 8166260aa839..418c9f64a9fd 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -31,6 +31,13 @@
 #define tty_ldisc_debug(tty, f, args...)
 #endif
 
+/* lockdep nested classes for tty->ldisc_sem */
+enum {
+        LDISC_SEM_NORMAL,
+        LDISC_SEM_OTHER,
+};
+
+
 /*
  *      This guards the refcounted line discipline lists. The lock
  *      must be taken with irqs off because there are hangup path
@@ -351,6 +358,86 @@ void tty_ldisc_deref(struct tty_ldisc *ld)
 }
 EXPORT_SYMBOL_GPL(tty_ldisc_deref);
 
+
+static inline int __lockfunc
+tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
+{
+        return ldsem_down_write(&tty->ldisc_sem, timeout);
+}
+
+static inline int __lockfunc
+tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
+{
+        return ldsem_down_write_nested(&tty->ldisc_sem,
+                                       LDISC_SEM_OTHER, timeout);
+}
+
+static inline void tty_ldisc_unlock(struct tty_struct *tty)
+{
+        return ldsem_up_write(&tty->ldisc_sem);
+}
+
+static int __lockfunc
+tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
+                            unsigned long timeout)
+{
+        int ret;
+
+        if (tty < tty2) {
+                ret = tty_ldisc_lock(tty, timeout);
+                if (ret) {
+                        ret = tty_ldisc_lock_nested(tty2, timeout);
+                        if (!ret)
+                                tty_ldisc_unlock(tty);
+                }
+        } else {
+                /* if this is possible, it has lots of implications */
+                WARN_ON_ONCE(tty == tty2);
+                if (tty2 && tty != tty2) {
+                        ret = tty_ldisc_lock(tty2, timeout);
+                        if (ret) {
+                                ret = tty_ldisc_lock_nested(tty, timeout);
+                                if (!ret)
+                                        tty_ldisc_unlock(tty2);
+                        }
+                } else
+                        ret = tty_ldisc_lock(tty, timeout);
+        }
+
+        if (!ret)
+                return -EBUSY;
+
+        set_bit(TTY_LDISC_HALTED, &tty->flags);
+        if (tty2)
+                set_bit(TTY_LDISC_HALTED, &tty2->flags);
+        return 0;
+}
+
+static void __lockfunc
+tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
+{
+        tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
+}
+
+static void __lockfunc tty_ldisc_unlock_pair(struct tty_struct *tty,
+                                             struct tty_struct *tty2)
+{
+        tty_ldisc_unlock(tty);
+        if (tty2)
+                tty_ldisc_unlock(tty2);
+}
+
+static void __lockfunc tty_ldisc_enable_pair(struct tty_struct *tty,
+                                             struct tty_struct *tty2)
+{
+        clear_bit(TTY_LDISC_HALTED, &tty->flags);
+        if (tty2)
+                clear_bit(TTY_LDISC_HALTED, &tty2->flags);
+
+        tty_ldisc_unlock_pair(tty, tty2);
+}
+
+
 /**
  *      tty_ldisc_enable        -       allow ldisc use
  *      @tty: terminal to activate ldisc on