[PATCH 2/6] tty, n_tty: Remove fasync() ldisc notification

From: Peter Hurley <peter@hurleysoftware.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>,
	linux-kernel@vger.kernel.org,
	Peter Hurley <peter@hurleysoftware.com>
Subject: [PATCH 2/6] tty, n_tty: Remove fasync() ldisc notification
Date: Sat, 12 Dec 2015 14:16:35 -0800	[thread overview]
Message-ID: <1449958599-5533-3-git-send-email-peter@hurleysoftware.com> (raw)
In-Reply-To: <1449958599-5533-1-git-send-email-peter@hurleysoftware.com>

Only the N_TTY line discipline implements the signal-driven i/o
notification enabled/disabled by fcntl(F_SETFL, O_ASYNC). The ldisc
fasync() notification is sent to the ldisc when the enable state has
changed (the tty core is notified via the fasync() VFS file operation).

The N_TTY line discipline used the enable state to change the wakeup
condition (minimum_to_wake = 1) for notifying the signal handler i/o is
available. However, just the presence of data is sufficient and necessary
to signal i/o is available, so changing minimum_to_wake is unnecessary
(and creates a race condition with read() and poll() which may be
concurrently updating minimum_to_wake).

Furthermore, since the kill_fasync() VFS helper performs no action if
the fasync list is empty, calling unconditionally is preferred; if
signal driven i/o just has been disabled, no signal will be sent by
kill_fasync() anyway so notification of the change via the ldisc
fasync() method is superfluous.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
 drivers/tty/n_tty.c       | 5 -----
 drivers/tty/tty_io.c      | 8 --------
 include/linux/tty_ldisc.h | 6 ------
 3 files changed, 19 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 16ba07b..e695f8f 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2448,10 +2448,6 @@ static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
 	}
 }
 
-static void n_tty_fasync(struct tty_struct *tty, int on)
-{
-}
-
 static void n_tty_closing(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
@@ -2472,7 +2468,6 @@ static struct tty_ldisc_ops n_tty_ops = {
 	.poll            = n_tty_poll,
 	.receive_buf     = n_tty_receive_buf,
 	.write_wakeup    = n_tty_write_wakeup,
-	.fasync		 = n_tty_fasync,
 	.receive_buf2	 = n_tty_receive_buf2,
 	.closing	 = n_tty_closing,
 };
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 5c8f519..df7a3a9 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2209,7 +2209,6 @@ static unsigned int tty_poll(struct file *filp, poll_table *wait)
 static int __tty_fasync(int fd, struct file *filp, int on)
 {
 	struct tty_struct *tty = file_tty(filp);
-	struct tty_ldisc *ldisc;
 	unsigned long flags;
 	int retval = 0;
 
@@ -2220,13 +2219,6 @@ static int __tty_fasync(int fd, struct file *filp, int on)
 	if (retval <= 0)
 		goto out;
 
-	ldisc = tty_ldisc_ref(tty);
-	if (ldisc) {
-		if (ldisc->ops->fasync)
-			ldisc->ops->fasync(tty, on);
-		tty_ldisc_deref(ldisc);
-	}
-
 	if (on) {
 		enum pid_type type;
 		struct pid *pid;
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index db0abe56..08fd06a 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -98,11 +98,6 @@
  *	seek to perform this action quickly but should wait until
  *	any pending driver I/O is completed.
  *
- * void (*fasync)(struct tty_struct *, int on)
- *
- *	Notify line discipline when signal-driven I/O is enabled or
- *	disabled.
- *
  * void (*dcd_change)(struct tty_struct *tty, unsigned int status)
  *
  *	Tells the discipline that the DCD pin has changed its status.
@@ -210,7 +205,6 @@ struct tty_ldisc_ops {
 			       char *fp, int count);
 	void	(*write_wakeup)(struct tty_struct *);
 	void	(*dcd_change)(struct tty_struct *, unsigned int);
-	void	(*fasync)(struct tty_struct *tty, int on);
 	int	(*receive_buf2)(struct tty_struct *, const unsigned char *cp,
 				char *fp, int count);
 	void	(*closing)(struct tty_struct *tty);
-- 
2.6.3

