tty: Convert tty_struct bitfield to ints

The stopped, hw_stopped, flow_stopped and packet bits are smp-unsafe
and interrupt-unsafe. For example,

CPU 0                         | CPU 1
                              |
tty->flow_stopped = 1         | tty->hw_stopped = 0

One of these updates will be corrupted, as the bitwise operation
on the bitfield is non-atomic.

Ensure each flag has a separate memory location, so concurrent
updates do not corrupt orthogonal states. Because DEC Alpha EV4 and EV5
cpus (from 1995) perform RMW on smaller-than-machine-word storage,
"separate memory location" must be int instead of byte.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 84132942902a..4cfd4a82fc31 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -261,7 +261,10 @@ struct tty_struct {
         unsigned long flags;
         int count;
         struct winsize winsize;         /* winsize_mutex */
-        unsigned char stopped:1, hw_stopped:1, flow_stopped:1, packet:1;
+        int stopped;
+        int flow_stopped;
+        int hw_stopped;
+        int packet;
         unsigned char ctrl_status;      /* ctrl_lock */
         unsigned int receive_room;      /* Bytes free for queue */
         int flow_change;