tty: fix sanity check

The WARN_ON() that was added to tty_reopen can be triggered in the specific
case of a hangup occurring during a re-open of a tty which is not in the
middle of being otherwise closed.

In that case however the WARN() is bogus as we don't hold the neccessary
locks to make a correct decision.

The case we should be checking is "if the ldisc is not changing and reopen
is occuring". We could drop the WARN_ON but for the moment the debug is more
valuable even if it means taking a mutex as it will find any other cases.

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

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 939e198d767..a3afa0c387c 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1263,7 +1263,9 @@ static int tty_reopen(struct tty_struct *tty)
         tty->count++;
         tty->driver = driver; /* N.B. why do this every time?? */
 
+        mutex_lock(&tty->ldisc_mutex);
         WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
+        mutex_unlock(&tty->ldisc_mutex);
 
         return 0;
 }