Prevent an O_NDELAY writer from blocking when a tty write is blocked by the tty atomic writer mutex

Without this a tty write could block if a previous blocking tty write was
in progress on the same tty and blocked by a line discipline or hardware
event.  Originally found and reported by Dave Johnson.

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index 154f42203b05..371631f4bfb9 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -1538,7 +1538,8 @@ static unsigned int normal_poll(struct tty_struct * tty, struct file * file, pol
                 else
                         tty->minimum_to_wake = 1;
         }
-        if (tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
+        if (!tty_is_writelocked(tty) &&
+                        tty->driver->chars_in_buffer(tty) < WAKEUP_CHARS &&
                         tty->driver->write_room(tty) > 0)
                 mask |= POLLOUT | POLLWRNORM;
         return mask;