linux-kernel - [PATCH v2 08/14] tty: Convert tty

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Message-Id: <1410375996-4693-9-git-send-email-peter@hurleysoftware.com>
Date:	Wed, 10 Sep 2014 15:06:30 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH v2 08/14] 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@...leysoftware.com>
---
 include/linux/tty.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 1c3316a..a707196 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;
-- 
2.1.0

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