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>

1 files changed, 25 insertions, 7 deletions

diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index 0c09409fa45d..001d9d875387 100644
--- a/drivers/char/n_tty.c
+++ b/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;
 }