tty: reorder ldisc locking

We need to release the BTM in paste_selection() when
sleeping in tty_ldisc_ref_wait to avoid deadlocks
with tty_ldisc_enable.

In tty_set_ldisc, we now always grab the BTM before
taking the ldisc_mutex in order to avoid AB-BA
deadlocks between the two.

tty_ldisc_halt potentially blocks on a workqueue
function that takes the BTM, so we must release
the BTM before calling it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 20 insertions, 4 deletions

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 97681ffd6cbd..0f494799da89 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -582,6 +582,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
         tty_wait_until_sent(tty, 0);
 
+        tty_lock();
         mutex_lock(&tty->ldisc_mutex);
 
         /*
@@ -591,13 +592,13 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
         while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
                 mutex_unlock(&tty->ldisc_mutex);
+                tty_unlock();
                 wait_event(tty_ldisc_wait,
                         test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
+                tty_lock();
                 mutex_lock(&tty->ldisc_mutex);
         }
 
-        tty_lock();
-
         set_bit(TTY_LDISC_CHANGING, &tty->flags);
 
         /*
@@ -634,8 +635,8 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
         flush_scheduled_work();
 
-        mutex_lock(&tty->ldisc_mutex);
         tty_lock();
+        mutex_lock(&tty->ldisc_mutex);
         if (test_bit(TTY_HUPPED, &tty->flags)) {
                 /* We were raced by the hangup method. It will have stomped
                    the ldisc data and closed the ldisc down */
@@ -782,7 +783,20 @@ void tty_ldisc_hangup(struct tty_struct *tty)
          * Avoid racing set_ldisc or tty_ldisc_release
          */
         mutex_lock(&tty->ldisc_mutex);
-        tty_ldisc_halt(tty);
+
+        /*
+         * this is like tty_ldisc_halt, but we need to give up
+         * the BTM before calling cancel_delayed_work_sync,
+         * which may need to wait for another function taking the BTM
+         */
+        clear_bit(TTY_LDISC, &tty->flags);
+        tty_unlock();
+        cancel_delayed_work_sync(&tty->buf.work);
+        mutex_unlock(&tty->ldisc_mutex);
+
+        tty_lock();
+        mutex_lock(&tty->ldisc_mutex);
+
         /* At this point we have a closed ldisc and we want to
            reopen it. We could defer this to the next open but
            it means auditing a lot of other paths so this is
@@ -853,8 +867,10 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
          * race with the set_ldisc code path.
          */
 
+        tty_unlock();
         tty_ldisc_halt(tty);
         flush_scheduled_work();
+        tty_lock();
 
         mutex_lock(&tty->ldisc_mutex);
         /*