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 <[e-mail]>
Signed-off-by: Peter Hurley <[e-mail]>
Acked-by: Ilya Zykov <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 				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)
···
 			disc->ops->receive_buf(tty, char_buf,
 							flag_buf, count);
 			spin_lock_irqsave(&buf->lock, flags);
+			/* 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);
 	}
 
-	/* 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);
-	}
 	spin_unlock_irqrestore(&buf->lock, flags);
 
 	tty_ldisc_deref(disc);