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 7f15b269cf3ef3ca2bdf723b65fdcfcaf23160bb..d4d71350a71d99b9579431dd43bec24e72154755 100644 (file)
--- 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;