staging: comedi: use a mutex when accessing driver list

Low-level comedi drivers registered with the comedi core by
`comedi_driver_register()` are linked together into a simple linked list
headed by the `comedi_drivers` variable and chained by the `next` member
of `struct comedi_driver`.  A driver is removed from the list by
`comedi_driver_unregister()`.  The driver list is iterated through by
`comedi_device_attach()` when the `COMEDI_DEVCONFIG` ioctl is used to
attach a "legacy" device to a driver, and is also iterated through by
`comedi_read()` in "proc.c" when reading "/proc/comedi".

There is currently no protection against items being added or removed
from the list while it is being iterated.  Add a mutex
`comedi_drivers_list_lock` to be locked while adding or removing an item
on the list, or when iterating through the list.

`comedi_driver_unregister()` also checks for and detaches any devices
using the driver.  This is currently done before unlinking the driver
from the list, but it makes more sense to unlink the driver from the
list first to prevent `comedi_device_attach()` attempting to use it, so
move the unlinking part to the start of the function.  Also, in
`comedi_device_attach()` hold on to the mutex until we've finished
attempting to attach the device to avoid it interfering with the
detachment in `comedi_driver_unregister()`.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_internal.h

@@ -24,6 +24,7 @@ extern unsigned int comedi_default_buf_maxsize_kb;
 /* drivers.c */
 
 extern struct comedi_driver *comedi_drivers;
+extern struct mutex comedi_drivers_list_lock;
 
 int insn_inval(struct comedi_device *, struct comedi_subdevice *,
 	       struct comedi_insn *, unsigned int *);

drivers/staging/comedi/drivers.c

@@ -38,6 +38,7 @@
 #include "comedi_internal.h"
 
 struct comedi_driver *comedi_drivers;
+DEFINE_MUTEX(comedi_drivers_list_lock);
 
 int comedi_set_hw_dev(struct comedi_device *dev, struct device *hw_dev)
 {
@@ -453,6 +454,7 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 	if (dev->attached)
 		return -EBUSY;
 
+	mutex_lock(&comedi_drivers_list_lock);
 	for (driv = comedi_drivers; driv; driv = driv->next) {
 		if (!try_module_get(driv->module))
 			continue;
@@ -473,7 +475,8 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 			comedi_report_boards(driv);
 			module_put(driv->module);
 		}
-		return -EIO;
+		ret = -EIO;
+		goto out;
 	}
 	if (driv->attach == NULL) {
 		/* driver does not support manual configuration */
@@ -481,7 +484,8 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 			 "driver '%s' does not support attach using comedi_config\n",
 			 driv->driver_name);
 		module_put(driv->module);
-		return -ENOSYS;
+		ret = -ENOSYS;
+		goto out;
 	}
 	/* initialize dev->driver here so
 	 * comedi_error() can be called from attach */
@@ -496,6 +500,8 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 		module_put(dev->driver->module);
 	}
 	/* On success, the driver module count has been incremented. */
+out:
+	mutex_unlock(&comedi_drivers_list_lock);
 	return ret;
 }
 
@@ -552,8 +558,10 @@ EXPORT_SYMBOL_GPL(comedi_auto_unconfig);
 
 int comedi_driver_register(struct comedi_driver *driver)
 {
+	mutex_lock(&comedi_drivers_list_lock);
 	driver->next = comedi_drivers;
 	comedi_drivers = driver;
+	mutex_unlock(&comedi_drivers_list_lock);
 
 	return 0;
 }
@@ -564,6 +572,20 @@ void comedi_driver_unregister(struct comedi_driver *driver)
 	struct comedi_driver *prev;
 	int i;
 
+	/* unlink the driver */
+	mutex_lock(&comedi_drivers_list_lock);
+	if (comedi_drivers == driver) {
+		comedi_drivers = driver->next;
+	} else {
+		for (prev = comedi_drivers; prev->next; prev = prev->next) {
+			if (prev->next == driver) {
+				prev->next = driver->next;
+				break;
+			}
+		}
+	}
+	mutex_unlock(&comedi_drivers_list_lock);
+
 	/* check for devices using this driver */
 	for (i = 0; i < COMEDI_NUM_BOARD_MINORS; i++) {
 		struct comedi_device *dev = comedi_dev_from_minor(i);
@@ -581,17 +603,5 @@ void comedi_driver_unregister(struct comedi_driver *driver)
 		}
 		mutex_unlock(&dev->mutex);
 	}
-
-	if (comedi_drivers == driver) {
-		comedi_drivers = driver->next;
-		return;
-	}
-
-	for (prev = comedi_drivers; prev->next; prev = prev->next) {
-		if (prev->next == driver) {
-			prev->next = driver->next;
-			return;
-		}
-	}
 }
 EXPORT_SYMBOL_GPL(comedi_driver_unregister);

drivers/staging/comedi/proc.c

@@ -55,6 +55,7 @@ static int comedi_read(struct seq_file *m, void *v)
 	if (!devices_q)
 		seq_puts(m, "no devices\n");
 
+	mutex_lock(&comedi_drivers_list_lock);
 	for (driv = comedi_drivers; driv; driv = driv->next) {
 		seq_printf(m, "%s:\n", driv->driver_name);
 		for (i = 0; i < driv->num_names; i++)
@@ -65,6 +66,7 @@ static int comedi_read(struct seq_file *m, void *v)
 		if (!driv->num_names)
 			seq_printf(m, " %s\n", driv->driver_name);
 	}
+	mutex_unlock(&comedi_drivers_list_lock);
 
 	return 0;
 }