tty: Fix race condition if flushing tty flip buffers

As Ilya Zykov identified in his patch 'PROBLEM: Race condition in
tty buffer's function flush_to_ldisc()', a race condition exists
which allows a parallel flush_to_ldisc() to flush and free the tty
flip buffers while those buffers are in-use. For example,

  CPU 0                         |  CPU 1                                  |  CPU 2
                                | flush_to_ldisc()                        |
                                |  grab spin lock                         |
tty_buffer_flush()              |                                         | flush_to_ldisc()
 wait for spin lock             |                                         |  wait for spin lock
                                |   if (!test_and_set_bit(TTYP_FLUSHING)) |
                                |    while (next flip buffer)             |
                                |     ...                                 |
                                |     drop spin lock                      |
  grab spin lock                |                                         |
   if (test_bit(TTYP_FLUSHING)) |                                         |
    set_bit(TTYP_FLUSHPENDING)  |      receive_buf()                      |
    drop spin lock              |                                         |
                                |                                         |   grab spin lock
                                |                                         |    if (!test_and_set_bit(TTYP_FLUSHING))
                                |                                         |    if (test_bit(TTYP_FLUSHPENDING))
                                |                                         |    __tty_buffer_flush()

CPU 2 has just flushed and freed all tty flip buffers while CPU 1 is
transferring data from the head flip buffer.

The original patch was rejected under the assumption that parallel
flush_to_ldisc() was not possible. Because of necessary changes to
the workqueue api, work items can execute in parallel on SMP.

This patch differs slightly from the original patch by testing for
a pending flush _after_ each receive_buf(), since TTYP_FLUSHPENDING
can only be set while the lock is dropped around receive_buf().

Reported-by: Ilya Zykov <linux@izyk.ru>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Ilya Zykov <linux@izyk.ru>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -449,11 +449,6 @@ static void flush_to_ldisc(struct work_struct *work)
 				tty_buffer_free(port, head);
 				continue;
 			}
-			/* Ldisc or user is trying to flush the buffers
-			   we are feeding to the ldisc, stop feeding the
-			   line discipline as we want to empty the queue */
-			if (test_bit(TTYP_FLUSHPENDING, &port->iflags))
-				break;
 			if (!tty->receive_room)
 				break;
 			if (count > tty->receive_room)
@@ -465,17 +460,20 @@ static void flush_to_ldisc(struct work_struct *work)
 			disc->ops->receive_buf(tty, char_buf,
 							flag_buf, count);
 			spin_lock_irqsave(&buf->lock, flags);
-		}
-		clear_bit(TTYP_FLUSHING, &port->iflags);
-	}
-
-	/* We may have a deferred request to flush the input buffer,
-	   if so pull the chain under the lock and empty the queue */
+			/* Ldisc or user is trying to flush the buffers.
+			   We may have a deferred request to flush the
+			   input buffer, if so pull the chain under the lock
+			   and empty the queue */
 			if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
 				__tty_buffer_flush(port);
 				clear_bit(TTYP_FLUSHPENDING, &port->iflags);
 				wake_up(&tty->read_wait);
+				break;
 			}
+		}
+		clear_bit(TTYP_FLUSHING, &port->iflags);
+	}
+
 	spin_unlock_irqrestore(&buf->lock, flags);
 
 	tty_ldisc_deref(disc);