[PATCH] uml: fix previous console locking

Eliminate the open_mutex after complaints from Blaisorblade.  It turns out
that the tty count provides the information needed to tell whether we are the
first opener or last closer.
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/line.c

@@ -425,42 +425,15 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data)
  * However, in this case, mconsole requests can come in "from the
  * side", and race with opens and closes.
  *
- * The problem comes from line_setup not wanting to sleep if
- * the device is open or being opened.  This can happen because the
- * first opener of a device is responsible for setting it up on the
- * host, and that can sleep.  The open of a port device will sleep
- * until someone telnets to it.
+ * mconsole config requests will want to be sure the device isn't in
+ * use, and get_config, open, and close will want a stable
+ * configuration.  The checking and modification of the configuration
+ * is done under a spinlock.  Checking whether the device is in use is
+ * line->tty->count > 1, also under the spinlock.
  *
- * The obvious solution of putting everything under a mutex fails
- * because then trying (and failing) to change the configuration of an
- * open(ing) device will block until the open finishes.  The right
- * thing to happen is for it to fail immediately.
- *
- * We can put the opening (and closing) of the host device under a
- * separate lock, but that has to be taken before the count lock is
- * released.  Otherwise, you open a window in which another open can
- * come through and assume that the host side is opened and working.
- *
- * So, if the tty count is one, open will take the open mutex
- * inside the count lock.  Otherwise, it just returns. This will sleep
- * if the last close is pending, and will block a setup or get_config,
- * but that should not last long.
- *
- * So, what we end up with is that open and close take the count lock.
- * If the first open or last close are happening, then the open mutex
- * is taken inside the count lock and the host opening or closing is done.
- *
- * setup and get_config only take the count lock.  setup modifies the
- * device configuration only if the open count is zero.  Arbitrarily
- * long blocking of setup doesn't happen because something would have to be
- * waiting for an open to happen.  However, a second open with
- * tty->count == 1 can't happen, and a close can't happen until the open
- * had finished.
- *
- * We can't maintain our own count here because the tty layer doesn't
- * match opens and closes.  It will call close if an open failed, and
- * a tty hangup will result in excess closes.  So, we rely on
- * tty->count instead.  It is one on both the first open and last close.
+ * tty->count serves to decide whether the device should be enabled or
+ * disabled on the host.  If it's equal to 1, then we are doing the
+ * first open or last close.  Otherwise, open and close just return.
  */
 
 int line_open(struct line *lines, struct tty_struct *tty)
@@ -476,7 +449,6 @@ int line_open(struct line *lines, struct tty_struct *tty)
 	if(tty->count > 1)
 		goto out_unlock;
 
-	mutex_lock(&line->open_mutex);
 	spin_unlock(&line->count_lock);
 
 	tty->driver_data = line;
@@ -493,7 +465,6 @@ int line_open(struct line *lines, struct tty_struct *tty)
 	chan_window_size(&line->chan_list, &tty->winsize.ws_row,
 			 &tty->winsize.ws_col);
 
-	mutex_unlock(&line->open_mutex);
 	return err;
 
 out_unlock:
@@ -523,7 +494,6 @@ void line_close(struct tty_struct *tty, struct file * filp)
 	if(tty->count > 1)
 		goto out_unlock;
 
-	mutex_lock(&line->open_mutex);
 	spin_unlock(&line->count_lock);
 
 	line->tty = NULL;
@@ -534,7 +504,6 @@ void line_close(struct tty_struct *tty, struct file * filp)
 		line->sigio = 0;
         }
 
-	mutex_unlock(&line->open_mutex);
 	return;
 
 out_unlock:
@@ -755,7 +724,6 @@ void lines_init(struct line *lines, int nlines, struct chan_opts *opts)
 	for(i = 0; i < nlines; i++){
 		line = &lines[i];
 		INIT_LIST_HEAD(&line->chan_list);
-		mutex_init(&line->open_mutex);
 
 		if(line->init_str == NULL)
 			continue;

arch/um/include/line.h

@@ -35,7 +35,6 @@ struct line {
 	spinlock_t count_lock;
 	int valid;
 
-	struct mutex open_mutex;
 	char *init_str;
 	int init_pri;
 	struct list_head chan_list;