linux-kernel - [PATCH 18/18] n_tty: Separate buffer indices to prevent cache-line sharing

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Date:	Tue, 19 Mar 2013 16:21:53 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Cc:	Min Zhang <mzhang@...sta.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH 18/18] n_tty: Separate buffer indices to prevent cache-line sharing

If the read buffer indices are in the same cache-line, cpus will
contended over the cache-line (so called 'false sharing').

Separate the producer-published fields from the consumer-published
fields; document the locks relevant to each field.

Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/n_tty.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index adccff8..9b828f0 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -86,28 +86,38 @@
 #define NO_ROOM		0
 
 struct n_tty_data {
-	unsigned int column;
+	/* producer-published */
+	size_t read_head;
+	size_t canon_head;
+	DECLARE_BITMAP(process_char_map, 256);
+
+	/* private to n_tty_receive_overrun (single-threaded) */
 	unsigned long overrun_time;
 	int num_overrun;
 
+	/* atomic */
 	unsigned long flags;
 
+	/* must hold exclusive termios_rwsem to reset these */
 	unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
 	unsigned char echo_overrun:1;
 
-	DECLARE_BITMAP(process_char_map, 256);
+	/* shared by producer and consumer */
+	char *read_buf;
 	DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);
 
-	char *read_buf;
-	size_t read_head;
-	size_t read_tail;
 	int minimum_to_wake;
 
+	/* consumer-published */
+	size_t read_tail;
+
+	/* protected by echo_lock */
 	unsigned char *echo_buf;
 	unsigned int echo_pos;
 	unsigned int echo_cnt;
 
-	size_t canon_head;
+	/* protected by output lock */
+	unsigned int column;
 	unsigned int canon_column;
 
 	struct mutex atomic_read_lock;
-- 
1.8.1.2

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