From: Peter Hurley <peter@hurleysoftware.com>
Date: Sat, 15 Jun 2013 13:14:30 +0000 (-0400)
Subject: n_tty: Separate buffer indices to prevent cache-line sharing
X-Git-Tag: MMI-PSA29.97-13-9~13777^2~204
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=fb7aa03db605e4f0b9a62cd4c77177c2596edd95;p=GitHub%2FMotorolaMobilityLLC%2Fkernel-slsi.git

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@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 a6eea30d0911..d0c8805d8131 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -82,29 +82,38 @@
 #endif
 
 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;
 
 	/* non-atomic */
 	bool no_room;
 
+	/* 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;