From: Peter Hurley <peter@hurleysoftware.com>
Date: Sat, 15 Jun 2013 14:04:29 +0000 (-0400)
Subject: n_tty: Avoid false-sharing echo buffer indices
X-Git-Tag: firefly_0821_release~176^2~5470^2~173
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=9dfd16ddea9bdbc8343340e543732db0a467ae32;p=firefly-linux-kernel-4.4.55.git

n_tty: Avoid false-sharing echo buffer indices

Separate the head & commit indices from the tail index to avoid
cache-line contention (so called 'false-sharing') between concurrent
threads.

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

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7f15b269cf3e..d4d71350a71d 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -90,6 +90,8 @@ struct n_tty_data {
 	/* producer-published */
 	size_t read_head;
 	size_t canon_head;
+	size_t echo_head;
+	size_t echo_commit;
 	DECLARE_BITMAP(process_char_map, 256);
 
 	/* private to n_tty_receive_overrun (single-threaded) */
@@ -105,20 +107,17 @@ struct n_tty_data {
 	/* shared by producer and consumer */
 	char *read_buf;
 	DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);
+	unsigned char *echo_buf;
 
 	int minimum_to_wake;
 
 	/* consumer-published */
 	size_t read_tail;
 
-	unsigned char *echo_buf;
-	size_t echo_head;
-	size_t echo_tail;
-	size_t echo_commit;
-
 	/* protected by output lock */
 	unsigned int column;
 	unsigned int canon_column;
+	size_t echo_tail;
 
 	struct mutex atomic_read_lock;
 	struct mutex output_lock;