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, 5 insertions, 4 deletions

diff --git a/drivers/char/n_hdlc.c b/drivers/char/n_hdlc.c
index 337a87f86a3b..37f7d3403040 100644
--- a/drivers/char/n_hdlc.c
+++ b/drivers/char/n_hdlc.c
@@ -780,13 +780,14 @@ static unsigned int n_hdlc_tty_poll(struct tty_struct *tty, struct file *filp,
                 poll_wait(filp, &tty->write_wait, wait);
 
                 /* set bits for operations that won't block */
-                if(n_hdlc->rx_buf_list.head)
+                if (n_hdlc->rx_buf_list.head)
                         mask |= POLLIN | POLLRDNORM;    /* readable */
                 if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                         mask |= POLLHUP;
-                if(tty_hung_up_p(filp))
+                if (tty_hung_up_p(filp))
                         mask |= POLLHUP;
-                if(n_hdlc->tx_free_buf_list.head)
+                if (!tty_is_writelocked(tty) &&
+                                n_hdlc->tx_free_buf_list.head)
                         mask |= POLLOUT | POLLWRNORM;   /* writable */
         }
         return mask;
@@ -861,7 +862,7 @@ static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
         spin_lock_irqsave(&list->spinlock,flags);
         
         buf->link=NULL;
-        if(list->tail)
+        if (list->tail)
                 list->tail->link = buf;
         else
                 list->head = buf;