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>

1 files changed, 15 insertions, 6 deletions

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;