tty: rewrite the ldisc locking

There are several pretty much unfixable races in the old ldisc code, especially
with respect to pty behaviour and also to hangup. It's easier to rewrite the
code than simply try and patch it up.

This patch
- splits the ldisc from the tty (so we will be able to refcount it more cleanly
  later)
- introduces a mutex lock for ldisc changing on an active device
- fixes the complete mess that hangup caused
- implements hopefully correct setldisc/close/hangup locking

There are still some problems around pty pairs that have always been there but
at least it is now possible to understand the code and fix further problems.

This fixes the following known bugs
- hang up can leak ldisc references
- hang up may not call open/close on ldisc in a matched way
- pty/tty pairs can deadlock during an ldisc change
- reading the ldisc proc files can cause every ldisc to be loaded

and probably a few other of the mysterious ldisc race reports.

I'm sure it also adds the odd new one.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index da2cb8c70c1..5acd29e6e04 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -110,7 +110,7 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf,
 	c = to->receive_room;
 	if (c > count)
 		c = count;
-	to->ldisc.ops->receive_buf(to, buf, NULL, c);
+	to->ldisc->ops->receive_buf(to, buf, NULL, c);
 
 	return c;
 }
@@ -148,11 +148,11 @@ static int pty_chars_in_buffer(struct tty_struct *tty)
 	int count;
 
 	/* We should get the line discipline lock for "tty->link" */
-	if (!to || !to->ldisc.ops->chars_in_buffer)
+	if (!to || !to->ldisc->ops->chars_in_buffer)
 		return 0;
 
 	/* The ldisc must report 0 if no characters available to be read */
-	count = to->ldisc.ops->chars_in_buffer(to);
+	count = to->ldisc->ops->chars_in_buffer(to);
 
 	if (tty->driver->subtype == PTY_TYPE_SLAVE)
 		return count;
@@ -186,8 +186,8 @@ static void pty_flush_buffer(struct tty_struct *tty)
 	if (!to)
 		return;
 
-	if (to->ldisc.ops->flush_buffer)
-		to->ldisc.ops->flush_buffer(to);
+	if (to->ldisc->ops->flush_buffer)
+		to->ldisc->ops->flush_buffer(to);
 
 	if (to->packet) {
 		spin_lock_irqsave(&tty->ctrl_lock, flags);