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>

drivers/bluetooth/hci_ldisc.c

@@ -277,8 +277,8 @@ static int hci_uart_tty_open(struct tty_struct *tty)
 	/* FIXME: why is this needed. Note don't use ldisc_ref here as the
 	   open path is before the ldisc is referencable */
 
-	if (tty->ldisc.ops->flush_buffer)
-		tty->ldisc.ops->flush_buffer(tty);
+	if (tty->ldisc->ops->flush_buffer)
+		tty->ldisc->ops->flush_buffer(tty);
 	tty_driver_flush_buffer(tty);
 
 	return 0;

drivers/char/cyclades.c

@@ -5200,7 +5200,7 @@ static int cyclades_proc_show(struct seq_file *m, void *v)
 					(cur_jifs - info->idle_stats.recv_idle)/
 					HZ, info->idle_stats.overruns,
 					/* FIXME: double check locking */
-					(long)info->port.tty->ldisc.ops->num);
+					(long)info->port.tty->ldisc->ops->num);
 			else
 				seq_printf(m, "%3d %8lu %10lu %8lu "
 					"%10lu %8lu %9lu %6ld\n",

drivers/char/epca.c

@@ -2114,8 +2114,8 @@ static int pc_ioctl(struct tty_struct *tty, struct file *file,
 			tty_wait_until_sent(tty, 0);
 		} else {
 			/* ldisc lock already held in ioctl */
-			if (tty->ldisc.ops->flush_buffer)
-				tty->ldisc.ops->flush_buffer(tty);
+			if (tty->ldisc->ops->flush_buffer)
+				tty->ldisc->ops->flush_buffer(tty);
 		}
 		unlock_kernel();
 		/* Fall Thru */

drivers/char/ip2/i2lib.c

@@ -868,11 +868,11 @@ i2Input(i2ChanStrPtr pCh)
 		amountToMove = count;
 	}
 	// Move the first block
-	pCh->pTTY->ldisc.ops->receive_buf( pCh->pTTY,
+	pCh->pTTY->ldisc->ops->receive_buf( pCh->pTTY,
 		 &(pCh->Ibuf[stripIndex]), NULL, amountToMove );
 	// If we needed to wrap, do the second data move
 	if (count > amountToMove) {
-		pCh->pTTY->ldisc.ops->receive_buf( pCh->pTTY,
+		pCh->pTTY->ldisc->ops->receive_buf( pCh->pTTY,
 		 pCh->Ibuf, NULL, count - amountToMove );
 	}
 	// Bump and wrap the stripIndex all at once by the amount of data read. This

drivers/char/ip2/ip2main.c

@@ -1315,8 +1315,8 @@ static inline void  isig(int sig, struct tty_struct *tty, int flush)
 	if (tty->pgrp)
 		kill_pgrp(tty->pgrp, sig, 1);
 	if (flush || !L_NOFLSH(tty)) {
-		if ( tty->ldisc.ops->flush_buffer )  
-			tty->ldisc.ops->flush_buffer(tty);
+		if ( tty->ldisc->ops->flush_buffer )  
+			tty->ldisc->ops->flush_buffer(tty);
 		i2InputFlush( tty->driver_data );
 	}
 }

drivers/char/n_hdlc.c

@@ -342,8 +342,8 @@ static int n_hdlc_tty_open (struct tty_struct *tty)
 #endif
 	
 	/* Flush any pending characters in the driver and discipline. */
-	if (tty->ldisc.ops->flush_buffer)
-		tty->ldisc.ops->flush_buffer(tty);
+	if (tty->ldisc->ops->flush_buffer)
+		tty->ldisc->ops->flush_buffer(tty);
 
 	tty_driver_flush_buffer(tty);
 		

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);

drivers/char/selection.c

@@ -327,7 +327,7 @@ int paste_selection(struct tty_struct *tty)
 		}
 		count = sel_buffer_lth - pasted;
 		count = min(count, tty->receive_room);
-		tty->ldisc.ops->receive_buf(tty, sel_buffer + pasted,
+		tty->ldisc->ops->receive_buf(tty, sel_buffer + pasted,
 								NULL, count);
 		pasted += count;
 	}

drivers/char/tty_io.c

@@ -491,22 +491,6 @@ void tty_ldisc_flush(struct tty_struct *tty)
 
 EXPORT_SYMBOL_GPL(tty_ldisc_flush);
 
-/**
- *	tty_reset_termios	-	reset terminal state
- *	@tty: tty to reset
- *
- *	Restore a terminal to the driver default state
- */
-
-static void tty_reset_termios(struct tty_struct *tty)
-{
-	mutex_lock(&tty->termios_mutex);
-	*tty->termios = tty->driver->init_termios;
-	tty->termios->c_ispeed = tty_termios_input_baud_rate(tty->termios);
-	tty->termios->c_ospeed = tty_termios_baud_rate(tty->termios);
-	mutex_unlock(&tty->termios_mutex);
-}
-
 /**
  *	do_tty_hangup		-	actual handler for hangup events
  *	@work: tty device
@@ -536,7 +520,6 @@ static void do_tty_hangup(struct work_struct *work)
 	struct file *cons_filp = NULL;
 	struct file *filp, *f = NULL;
 	struct task_struct *p;
-	struct tty_ldisc *ld;
 	int    closecount = 0, n;
 	unsigned long flags;
 	int refs = 0;
@@ -567,40 +550,8 @@ static void do_tty_hangup(struct work_struct *work)
 		filp->f_op = &hung_up_tty_fops;
 	}
 	file_list_unlock();
-	/*
-	 * FIXME! What are the locking issues here? This may me overdoing
-	 * things... This question is especially important now that we've
-	 * removed the irqlock.
-	 */
-	ld = tty_ldisc_ref(tty);
-	if (ld != NULL) {
-		/* We may have no line discipline at this point */
-		if (ld->ops->flush_buffer)
-			ld->ops->flush_buffer(tty);
-		tty_driver_flush_buffer(tty);
-		if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
-		    ld->ops->write_wakeup)
-			ld->ops->write_wakeup(tty);
-		if (ld->ops->hangup)
-			ld->ops->hangup(tty);
-	}
-	/*
-	 * FIXME: Once we trust the LDISC code better we can wait here for
-	 * ldisc completion and fix the driver call race
-	 */
-	wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
-	wake_up_interruptible_poll(&tty->read_wait, POLLIN);
-	/*
-	 * Shutdown the current line discipline, and reset it to
-	 * N_TTY.
-	 */
-	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
-		tty_reset_termios(tty);
-	/* Defer ldisc switch */
-	/* tty_deferred_ldisc_switch(N_TTY);
 
-	  This should get done automatically when the port closes and
-	  tty_release is called */
+	tty_ldisc_hangup(tty);
 
 	read_lock(&tasklist_lock);
 	if (tty->session) {
@@ -629,12 +580,15 @@ static void do_tty_hangup(struct work_struct *work)
 	read_unlock(&tasklist_lock);
 
 	spin_lock_irqsave(&tty->ctrl_lock, flags);
-	tty->flags = 0;
+	clear_bit(TTY_THROTTLED, &tty->flags);
+	clear_bit(TTY_PUSH, &tty->flags);
+	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 	put_pid(tty->session);
 	put_pid(tty->pgrp);
 	tty->session = NULL;
 	tty->pgrp = NULL;
 	tty->ctrl_status = 0;
+	set_bit(TTY_HUPPED, &tty->flags);
 	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 
 	/* Account for the p->signal references we killed */
@@ -660,10 +614,7 @@ static void do_tty_hangup(struct work_struct *work)
 	 * can't yet guarantee all that.
 	 */
 	set_bit(TTY_HUPPED, &tty->flags);
-	if (ld) {
 	tty_ldisc_enable(tty);
-		tty_ldisc_deref(ld);
-	}
 	unlock_kernel();
 	if (f)
 		fput(f);
@@ -2570,7 +2521,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case TIOCGSID:
 		return tiocgsid(tty, real_tty, p);
 	case TIOCGETD:
-		return put_user(tty->ldisc.ops->num, (int __user *)p);
+		return put_user(tty->ldisc->ops->num, (int __user *)p);
 	case TIOCSETD:
 		return tiocsetd(tty, p);
 	/*
@@ -2785,6 +2736,7 @@ void initialize_tty_struct(struct tty_struct *tty,
 	tty->buf.head = tty->buf.tail = NULL;
 	tty_buffer_init(tty);
 	mutex_init(&tty->termios_mutex);
+	mutex_init(&tty->ldisc_mutex);
 	init_waitqueue_head(&tty->write_wait);
 	init_waitqueue_head(&tty->read_wait);
 	INIT_WORK(&tty->hangup_work, do_tty_hangup);

include/linux/tty.h

@@ -226,8 +226,11 @@ struct tty_struct {
 	struct tty_driver *driver;
 	const struct tty_operations *ops;
 	int index;
-	/* The ldisc objects are protected by tty_ldisc_lock at the moment */
-	struct tty_ldisc ldisc;
+
+	/* Protects ldisc changes: Lock tty not pty */
+	struct mutex ldisc_mutex;
+	struct tty_ldisc *ldisc;
+
 	struct mutex termios_mutex;
 	spinlock_t ctrl_lock;
 	/* Termios values are protected by the termios mutex */
@@ -314,6 +317,7 @@ struct tty_struct {
 #define TTY_CLOSING 		7	/* ->close() in progress */
 #define TTY_LDISC 		9	/* Line discipline attached */
 #define TTY_LDISC_CHANGING 	10	/* Line discipline changing */
+#define TTY_LDISC_OPEN	 	11	/* Line discipline is open */
 #define TTY_HW_COOK_OUT 	14	/* Hardware can do output cooking */
 #define TTY_HW_COOK_IN 		15	/* Hardware can do input cooking */
 #define TTY_PTY_LOCK 		16	/* pty private */
@@ -406,6 +410,7 @@ extern int tty_termios_hw_change(struct ktermios *a, struct ktermios *b);
 extern struct tty_ldisc *tty_ldisc_ref(struct tty_struct *);
 extern void tty_ldisc_deref(struct tty_ldisc *);
 extern struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *);
+extern void tty_ldisc_hangup(struct tty_struct *tty);
 extern const struct file_operations tty_ldiscs_proc_fops;
 
 extern void tty_wakeup(struct tty_struct *tty);