linux-kernel - [PATCH 5/9] n_tty: Eliminate echo

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Date:	Mon, 15 Apr 2013 11:29:43 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH 5/9] n_tty: Eliminate echo_commit memory barrier

Use output_lock mutex as a memory barrier when storing echo_commit.

Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/n_tty.c | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 49ef008..c101492 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -637,21 +637,16 @@ break_out:
  *	are prioritized.  Also, when control characters are echoed with a
  *	prefixed "^", the pair is treated atomically and thus not separated.
  *
- *	Locking: output_lock to protect column state and space left
+ *	Locking: callers must hold output_lock
  */
 
-static void process_echoes(struct tty_struct *tty)
+static void __process_echoes(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	int	space, nr;
 	size_t tail;
 	unsigned char c;
 
-	if (ldata->echo_commit == ldata->echo_tail)
-		return;
-
-	mutex_lock(&ldata->output_lock);
-
 	space = tty_write_room(tty);
 
 	tail = ldata->echo_tail;
@@ -772,20 +767,34 @@ static void process_echoes(struct tty_struct *tty)
 	}
 
 	ldata->echo_tail = tail;
+}
+
+static void commit_echoes(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
 
+	mutex_lock(&ldata->output_lock);
+	ldata->echo_commit = ldata->echo_head;
+	__process_echoes(tty);
 	mutex_unlock(&ldata->output_lock);
 
 	if (tty->ops->flush_chars)
 		tty->ops->flush_chars(tty);
 }
 
-static void commit_echoes(struct tty_struct *tty)
+static void process_echoes(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 
-	smp_mb();
-	ldata->echo_commit = ldata->echo_head;
-	process_echoes(tty);
+	if (!L_ECHO(tty) || ldata->echo_commit == ldata->echo_tail)
+		return;
+
+	mutex_lock(&ldata->output_lock);
+	__process_echoes(tty);
+	mutex_unlock(&ldata->output_lock);
+
+	if (tty->ops->flush_chars)
+		tty->ops->flush_chars(tty);
 }
 
 /**
-- 
1.8.1.2

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