firewire: fix "kobject_add failed for fw* with -EEXIST"

There is a race between shutdown and creation of devices:  fw-core may
attempt to add a device with the same name of an already existing
device.  http://bugzilla.kernel.org/show_bug.cgi?id=9828

Impact of the bug:  Happens rarely (when shutdown of a device coincides
with creation of another), forces the user to unplug and replug the new
device to get it working.

The fix is obvious:  Free the minor number *after* instead of *before*
device_unregister().  This requires to take an additional reference of
the fw_device as long as the IDR tree points to it.

And while we are at it, we fix an additional race condition:
fw_device_op_open() took its reference of the fw_device a little bit too
late, hence was in danger to access an already invalid fw_device.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

1 files changed, 14 insertions, 6 deletions

diff --git a/drivers/firewire/fw-device.c b/drivers/firewire/fw-device.c
index de9066e69ad..c04c28800f1 100644
--- a/drivers/firewire/fw-device.c
+++ b/drivers/firewire/fw-device.c
@@ -610,12 +610,14 @@ static DECLARE_RWSEM(idr_rwsem);
 static DEFINE_IDR(fw_device_idr);
 int fw_cdev_major;
 
-struct fw_device *fw_device_from_devt(dev_t devt)
+struct fw_device *fw_device_get_by_devt(dev_t devt)
 {
         struct fw_device *device;
 
         down_read(&idr_rwsem);
         device = idr_find(&fw_device_idr, MINOR(devt));
+        if (device)
+                fw_device_get(device);
         up_read(&idr_rwsem);
 
         return device;
@@ -627,13 +629,14 @@ static void fw_device_shutdown(struct work_struct *work)
                 container_of(work, struct fw_device, work.work);
         int minor = MINOR(device->device.devt);
 
-        down_write(&idr_rwsem);
-        idr_remove(&fw_device_idr, minor);
-        up_write(&idr_rwsem);
-
         fw_device_cdev_remove(device);
         device_for_each_child(&device->device, NULL, shutdown_unit);
         device_unregister(&device->device);
+
+        down_write(&idr_rwsem);
+        idr_remove(&fw_device_idr, minor);
+        up_write(&idr_rwsem);
+        fw_device_put(device);
 }
 
 static struct device_type fw_device_type = {
@@ -682,10 +685,13 @@ static void fw_device_init(struct work_struct *work)
         }
 
         err = -ENOMEM;
+
+        fw_device_get(device);
         down_write(&idr_rwsem);
         if (idr_pre_get(&fw_device_idr, GFP_KERNEL))
                 err = idr_get_new(&fw_device_idr, device, &minor);
         up_write(&idr_rwsem);
+
         if (err < 0)
                 goto error;
 
@@ -741,7 +747,9 @@ static void fw_device_init(struct work_struct *work)
         idr_remove(&fw_device_idr, minor);
         up_write(&idr_rwsem);
  error:
-        put_device(&device->device);
+        fw_device_put(device);          /* fw_device_idr's reference */
+
+        put_device(&device->device);    /* our reference */
 }
 
 static int update_unit(struct device *dev, void *data)