[PATCH] uml: console locking fixes

Clean up the console driver locking.  There are various problems here,
including sleeping under a spinlock and spinlock recursion, some of which are
fixed here.  This patch deals with the locking involved with opens and closes.
 The problem is that an mconsole request to change a console's configuration
can race with an open.  Changing a configuration should only be done when a
console isn't opened.  Also, an open must be looking at a stable
configuration.  In addition, a get configuration request must observe the same
locking since it must also see a stable configuration.  With the old locking,
it was possible for this to hang indefinitely in some cases because open would
block for a long time waiting for a connection from the host while holding the
lock needed by the mconsole request.

As explained in the long comment, this is fixed by adding a spinlock for the
use count and configuration and a mutex for the actual open and close.

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>

1 files changed, 122 insertions, 64 deletions

diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 83301e1ef67c..799fca3644e9 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -191,7 +191,6 @@ void line_flush_buffer(struct tty_struct *tty)
         /*XXX: copied from line_write, verify if it is correct!*/
         if(tty->stopped)
                 return;
-                //return 0;
 
         spin_lock_irqsave(&line->lock, flags);
         err = flush_buffer(line);
@@ -421,42 +420,84 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data)
         return err;
 }
 
+/* Normally, a driver like this can rely mostly on the tty layer
+ * locking, particularly when it comes to the driver structure.
+ * 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.
+ *
+ * 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.
+ */
+
 int line_open(struct line *lines, struct tty_struct *tty)
 {
-        struct line *line;
+        struct line *line = &lines[tty->index];
         int err = -ENODEV;
 
-        line = &lines[tty->index];
-        tty->driver_data = line;
+        spin_lock(&line->count_lock);
+        if(!line->valid)
+                goto out_unlock;
+
+        err = 0;
+        if(tty->count > 1)
+                goto out_unlock;
 
-        /* The IRQ which takes this lock is not yet enabled and won't be run
-         * before the end, so we don't need to use spin_lock_irq.*/
-        spin_lock(&line->lock);
+        mutex_lock(&line->open_mutex);
+        spin_unlock(&line->count_lock);
 
         tty->driver_data = line;
         line->tty = tty;
-        if(!line->valid)
-                goto out;
 
-        if(tty->count == 1){
-                /* Here the device is opened, if necessary, and interrupt
-                 * is registered.
-                 */
-                enable_chan(line);
-                INIT_DELAYED_WORK(&line->task, line_timer_cb);
-
-                if(!line->sigio){
-                        chan_enable_winch(&line->chan_list, tty);
-                        line->sigio = 1;
-                }
+        enable_chan(line);
+        INIT_DELAYED_WORK(&line->task, line_timer_cb);
 
-                chan_window_size(&line->chan_list, &tty->winsize.ws_row,
-                                 &tty->winsize.ws_col);
+        if(!line->sigio){
+                chan_enable_winch(&line->chan_list, tty);
+                line->sigio = 1;
         }
 
-        err = 0;
-out:
-        spin_unlock(&line->lock);
+        chan_window_size(&line->chan_list, &tty->winsize.ws_row,
+                         &tty->winsize.ws_col);
+
+        mutex_unlock(&line->open_mutex);
+        return err;
+
+out_unlock:
+        spin_unlock(&line->count_lock);
         return err;
 }
 
@@ -466,25 +507,38 @@ void line_close(struct tty_struct *tty, struct file * filp)
 {
         struct line *line = tty->driver_data;
 
-        /* XXX: I assume this should be called in process context, not with
-         *  interrupts disabled!
-         */
-        spin_lock_irq(&line->lock);
+        /* If line_open fails (and tty->driver_data is never set),
+         * tty_open will call line_close.  So just return in this case.
+         */
+        if(line == NULL)
+                return;
 
         /* We ignore the error anyway! */
         flush_buffer(line);
 
-        if(tty->count == 1){
-                line->tty = NULL;
-                tty->driver_data = NULL;
+        spin_lock(&line->count_lock);
+        if(!line->valid)
+                goto out_unlock;
+
+        if(tty->count > 1)
+                goto out_unlock;
 
-                if(line->sigio){
-                        unregister_winch(tty);
-                        line->sigio = 0;
-                }
+        mutex_lock(&line->open_mutex);
+        spin_unlock(&line->count_lock);
+
+        line->tty = NULL;
+        tty->driver_data = NULL;
+
+        if(line->sigio){
+                unregister_winch(tty);
+                line->sigio = 0;
         }
 
-        spin_unlock_irq(&line->lock);
+        mutex_unlock(&line->open_mutex);
+        return;
+
+out_unlock:
+        spin_unlock(&line->count_lock);
 }
 
 void close_lines(struct line *lines, int nlines)
@@ -495,6 +549,30 @@ void close_lines(struct line *lines, int nlines)
                 close_chan(&lines[i].chan_list, 0);
 }
 
+static void setup_one_line(struct line *lines, int n, char *init, int init_prio)
+{
+        struct line *line = &lines[n];
+
+        spin_lock(&line->count_lock);
+
+        if(line->tty != NULL){
+                printk("line_setup - device %d is open\n", n);
+                goto out;
+        }
+
+        if (line->init_pri <= init_prio){
+                line->init_pri = init_prio;
+                if (!strcmp(init, "none"))
+                        line->valid = 0;
+                else {
+                        line->init_str = init;
+                        line->valid = 1;
+                }
+        }
+out:
+        spin_unlock(&line->count_lock);
+}
+
 /* Common setup code for both startup command line and mconsole initialization.
  * @lines contains the array (of size @num) to modify;
  * @init is the setup string;
@@ -526,32 +604,11 @@ int line_setup(struct line *lines, unsigned int num, char *init)
                        n, num - 1);
                 return 0;
         }
-        else if (n >= 0){
-                if (lines[n].tty != NULL) {
-                        printk("line_setup - device %d is open\n", n);
-                        return 0;
-                }
-                if (lines[n].init_pri <= INIT_ONE){
-                        lines[n].init_pri = INIT_ONE;
-                        if (!strcmp(init, "none"))
-                                lines[n].valid = 0;
-                        else {
-                                lines[n].init_str = init;
-                                lines[n].valid = 1;
-                        }
-                }
-        }
+        else if (n >= 0)
+                setup_one_line(lines, n, init, INIT_ONE);
         else {
-                for(i = 0; i < num; i++){
-                        if(lines[i].init_pri <= INIT_ALL){
-                                lines[i].init_pri = INIT_ALL;
-                                if(!strcmp(init, "none")) lines[i].valid = 0;
-                                else {
-                                        lines[i].init_str = init;
-                                        lines[i].valid = 1;
-                                }
-                        }
-                }
+                for(i = 0; i < num; i++)
+                        setup_one_line(lines, i, init, INIT_ALL);
         }
         return n == -1 ? num : n;
 }
@@ -602,13 +659,13 @@ int line_get_config(char *name, struct line *lines, unsigned int num, char *str,
 
         line = &lines[dev];
 
-        spin_lock(&line->lock);
+        spin_lock(&line->count_lock);
         if(!line->valid)
                 CONFIG_CHUNK(str, size, n, "none", 1);
         else if(line->tty == NULL)
                 CONFIG_CHUNK(str, size, n, line->init_str, 1);
         else n = chan_config_string(&line->chan_list, str, size, error_out);
-        spin_unlock(&line->lock);
+        spin_unlock(&line->count_lock);
 
         return n;
 }
@@ -688,6 +745,7 @@ 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;