tty: BKL pushdown

- Push the BKL down into the line disciplines
- Switch the tty layer to unlocked_ioctl
- Introduce a new ctrl_lock spin lock for the control bits
- Eliminate much of the lock_kernel use in n_tty
- Prepare to (but don't yet) call the drivers with the lock dropped
  on the paths that historically held the lock

BKL now primarily protects open/close/ldisc change in the tty layer

[jirislaby@gmail.com: a couple of fixes]
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -183,22 +183,24 @@ static void reset_buffer_flags(struct tty_struct *tty)
  *	at hangup) or when the N_TTY line discipline internally has to
  *	clean the pending queue (for example some signals).
  *
- *	FIXME: tty->ctrl_status is not spinlocked and relies on
- *	lock_kernel() still.
+ *	Locking: ctrl_lock
  */
 
 static void n_tty_flush_buffer(struct tty_struct *tty)
 {
+	unsigned long flags;
 	/* clear everything and unthrottle the driver */
 	reset_buffer_flags(tty);
 
 	if (!tty->link)
 		return;
 
+	spin_lock_irqsave(&tty->ctrl_lock, flags);
 	if (tty->link->packet) {
 		tty->ctrl_status |= TIOCPKT_FLUSHREAD;
 		wake_up_interruptible(&tty->link->read_wait);
 	}
+	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 }
 
 /**
@@ -264,7 +266,7 @@ static inline int is_continuation(unsigned char c, struct tty_struct *tty)
  *	relevant in the world today. If you ever need them, add them here.
  *
  *	Called from both the receive and transmit sides and can be called
- *	re-entrantly. Relies on lock_kernel() still.
+ *	re-entrantly. Relies on lock_kernel() for tty->column state.
  */
 
 static int opost(unsigned char c, struct tty_struct *tty)
@@ -275,6 +277,7 @@ static int opost(unsigned char c, struct tty_struct *tty)
 	if (!space)
 		return -1;
 
+	lock_kernel();
 	if (O_OPOST(tty)) {
 		switch (c) {
 		case '\n':
@@ -323,6 +326,7 @@ static int opost(unsigned char c, struct tty_struct *tty)
 		}
 	}
 	tty->driver->put_char(tty, c);
+	unlock_kernel();
 	return 0;
 }
 
@@ -337,7 +341,8 @@ static int opost(unsigned char c, struct tty_struct *tty)
  *	the simple cases normally found and helps to generate blocks of
  *	symbols for the console driver and thus improve performance.
  *
- *	Called from write_chan under the tty layer write lock.
+ *	Called from write_chan under the tty layer write lock. Relies
+ *	on lock_kernel for the tty->column state.
  */
 
 static ssize_t opost_block(struct tty_struct *tty,
@@ -353,6 +358,7 @@ static ssize_t opost_block(struct tty_struct *tty,
 	if (nr > space)
 		nr = space;
 
+	lock_kernel();
 	for (i = 0, cp = buf; i < nr; i++, cp++) {
 		switch (*cp) {
 		case '\n':
@@ -387,6 +393,7 @@ break_out:
 	if (tty->driver->flush_chars)
 		tty->driver->flush_chars(tty);
 	i = tty->driver->write(tty, buf, i);
+	unlock_kernel();
 	return i;
 }
 
@@ -1194,6 +1201,11 @@ extern ssize_t redirected_tty_write(struct file *, const char __user *,
  *	Perform job control management checks on this file/tty descriptor
  *	and if appropriate send any needed signals and return a negative
  *	error code if action should be taken.
+ *
+ *	FIXME:
+ *	Locking: None - redirected write test is safe, testing
+ *	current->signal should possibly lock current->sighand
+ *	pgrp locking ?
  */
 
 static int job_control(struct tty_struct *tty, struct file *file)
@@ -1246,6 +1258,7 @@ static ssize_t read_chan(struct tty_struct *tty, struct file *file,
 	ssize_t size;
 	long timeout;
 	unsigned long flags;
+	int packet;
 
 do_it_again:
 
@@ -1289,16 +1302,19 @@ do_it_again:
 		if (mutex_lock_interruptible(&tty->atomic_read_lock))
 			return -ERESTARTSYS;
 	}
+	packet = tty->packet;
 
 	add_wait_queue(&tty->read_wait, &wait);
 	while (nr) {
 		/* First test for status change. */
-		if (tty->packet && tty->link->ctrl_status) {
+		if (packet && tty->link->ctrl_status) {
 			unsigned char cs;
 			if (b != buf)
 				break;
+			spin_lock_irqsave(&tty->link->ctrl_lock, flags);
 			cs = tty->link->ctrl_status;
 			tty->link->ctrl_status = 0;
+			spin_unlock_irqrestore(&tty->link->ctrl_lock, flags);
 			if (tty_put_user(tty, cs, b++)) {
 				retval = -EFAULT;
 				b--;
@@ -1333,6 +1349,7 @@ do_it_again:
 				retval = -ERESTARTSYS;
 				break;
 			}
+			/* FIXME: does n_tty_set_room need locking ? */
 			n_tty_set_room(tty);
 			timeout = schedule_timeout(timeout);
 			continue;
@@ -1340,7 +1357,7 @@ do_it_again:
 		__set_current_state(TASK_RUNNING);
 
 		/* Deal with packet mode. */
-		if (tty->packet && b == buf) {
+		if (packet && b == buf) {
 			if (tty_put_user(tty, TIOCPKT_DATA, b++)) {
 				retval = -EFAULT;
 				b--;
@@ -1388,6 +1405,8 @@ do_it_again:
 				break;
 		} else {
 			int uncopied;
+			/* The copy function takes the read lock and handles
+			   locking internally for this case */
 			uncopied = copy_from_read_buf(tty, &b, &nr);
 			uncopied += copy_from_read_buf(tty, &b, &nr);
 			if (uncopied) {
@@ -1429,7 +1448,6 @@ do_it_again:
 		 goto do_it_again;
 
 	n_tty_set_room(tty);
-
 	return retval;
 }