[PATCH] Fix race condition in hvc console.

tty_schedule_flip() would schedule a thread that would call flush_to_ldisc().
If tty_buffer_request_room() gets called prior to that thread running --
which is likely in this loop in hvc_poll(), it would set the active flag
in the tty buffer and consequently flush_to_ldisc() would ignore it.

The result is that input on the hvc console is not processed.

This fix calls tty_flip_buffer_push (and flags the tty as
"low_latency").  The push to the ldisc thus happens synchronously.
Signed-off-by: NMichal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

drivers/char/hvc_console.c

@@ -335,6 +335,8 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
 	} /* else count == 0 */
 
 	tty->driver_data = hp;
+	tty->low_latency = 1; /* Makes flushes to ldisc synchronous. */
+
 	hp->tty = tty;
 	/* Save for request_irq outside of spin_lock. */
 	irq = hp->irq;
@@ -633,9 +635,6 @@ static int hvc_poll(struct hvc_struct *hp)
 			tty_insert_flip_char(tty, buf[i], 0);
 		}
 
-		if (count)
-			tty_schedule_flip(tty);
-
 		/*
 		 * Account for the total amount read in one loop, and if above
 		 * 64 bytes, we do a quick schedule loop to let the tty grok
@@ -656,6 +655,9 @@ static int hvc_poll(struct hvc_struct *hp)
  bail:
 	spin_unlock_irqrestore(&hp->lock, flags);
 
+	if (read_total)
+		tty_flip_buffer_push(tty);
+	
 	return poll_mask;
 }