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 a6eea30d0..d0c8805 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;