#include "dm.h"
  
  /*
   * The kobject release method must not be placed in the module itself,
   * otherwise we are subject to module unload races.
   *
   * The release method is called when the last reference to the kobject is
   * dropped. It may be called by any other kernel code that drops the last
   * reference.
   *
   * The release method suffers from module unload race. We may prevent the
   * module from being unloaded at the start of the release method (using
   * increased module reference count or synchronizing against the release
   * method), however there is no way to prevent the module from being
   * unloaded at the end of the release method.
   *
   * If this code were placed in the dm module, the following race may
   * happen:
   *  1. Some other process takes a reference to dm kobject
   *  2. The user issues ioctl function to unload the dm device
   *  3. dm_sysfs_exit calls kobject_put, however the object is not released
   *     because of the other reference taken at step 1
   *  4. dm_sysfs_exit waits on the completion
   *  5. The other process that took the reference in step 1 drops it,
   *     dm_kobject_release is called from this process
   *  6. dm_kobject_release calls complete()
   *  7. a reschedule happens before dm_kobject_release returns
   *  8. dm_sysfs_exit continues, the dm device is unloaded, module reference
   *     count is decremented
   *  9. The user unloads the dm module
   * 10. The other process that was rescheduled in step 7 continues to run,
   *     it is now executing code in unloaded module, so it crashes
   *
   * Note that if the process that takes the foreign reference to dm kobject
   * has a low priority and the system is sufficiently loaded with
   * higher-priority processes that prevent the low-priority process from
   * being scheduled long enough, this bug may really happen.
   *
   * In order to fix this module unload race, we place the release method
   * into a helper code that is compiled directly into the kernel.
   */
  
  void dm_kobject_release(struct kobject *kobj)
  {
  	complete(dm_get_completion_from_kobject(kobj));
  }
  
  EXPORT_SYMBOL(dm_kobject_release);