Driver core: driver_find() drops reference before returning

As part of the removal of get_driver()/put_driver(), this patch
(as1510) changes driver_find(); it now drops the reference it acquires
before returning.  The patch also adjusts all the callers of
driver_find() to remove the now unnecessary calls to put_driver().

In addition, the patch adds a warning to driver_find(): Callers must
make sure the driver they are searching for does not get unloaded
while they are using it.  This has always been the case; driver_find()
has never prevented a driver from being unregistered or unloaded.
Hence the patch will not introduce any new bugs.  The existing callers
all seem to be okay in this respect, however I don't understand the
video drivers well enough to be certain about them.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Dmitry Torokhov <dmitry.torokhov@gmail.com>
CC: Kyungmin Park <kyungmin.park@samsung.com>
CC: Andy Walls <awalls@md.metrocast.net>
CC: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 5 insertions, 2 deletions

diff --git a/drivers/base/driver.c b/drivers/base/driver.c
index b631f7c59453..e979cad75c6e 100644
--- a/drivers/base/driver.c
+++ b/drivers/base/driver.c
@@ -234,7 +234,6 @@ int driver_register(struct device_driver *drv)
 
         other = driver_find(drv->name, drv->bus);
         if (other) {
-                put_driver(other);
                 printk(KERN_ERR "Error: Driver '%s' is already registered, "
                         "aborting...\n", drv->name);
                 return -EBUSY;
@@ -275,7 +274,9 @@ EXPORT_SYMBOL_GPL(driver_unregister);
  * Call kset_find_obj() to iterate over list of drivers on
  * a bus to find driver by name. Return driver if found.
  *
- * Note that kset_find_obj increments driver's reference count.
+ * This routine provides no locking to prevent the driver it returns
+ * from being unregistered or unloaded while the caller is using it.
+ * The caller is responsible for preventing this.
  */
 struct device_driver *driver_find(const char *name, struct bus_type *bus)
 {
@@ -283,6 +284,8 @@ struct device_driver *driver_find(const char *name, struct bus_type *bus)
         struct driver_private *priv;
 
         if (k) {
+                /* Drop reference added by kset_find_obj() */
+                kobject_put(k);
                 priv = to_driver(k);
                 return priv->driver;
         }