drivers/md/dm-builtin.c

   1 #include "dm.h"
   2
   3 /*
   4  * The kobject release method must not be placed in the module itself,
   5  * otherwise we are subject to module unload races.
   6  *
   7  * The release method is called when the last reference to the kobject is
   8  * dropped. It may be called by any other kernel code that drops the last
   9  * reference.
  10  *
  11  * The release method suffers from module unload race. We may prevent the
  12  * module from being unloaded at the start of the release method (using
  13  * increased module reference count or synchronizing against the release
  14  * method), however there is no way to prevent the module from being
  15  * unloaded at the end of the release method.
  16  *
  17  * If this code were placed in the dm module, the following race may
  18  * happen:
  19  *  1. Some other process takes a reference to dm kobject
  20  *  2. The user issues ioctl function to unload the dm device
  21  *  3. dm_sysfs_exit calls kobject_put, however the object is not released
  22  *     because of the other reference taken at step 1
  23  *  4. dm_sysfs_exit waits on the completion
  24  *  5. The other process that took the reference in step 1 drops it,
  25  *     dm_kobject_release is called from this process
  26  *  6. dm_kobject_release calls complete()
  27  *  7. a reschedule happens before dm_kobject_release returns
  28  *  8. dm_sysfs_exit continues, the dm device is unloaded, module reference
  29  *     count is decremented
  30  *  9. The user unloads the dm module
  31  * 10. The other process that was rescheduled in step 7 continues to run,
  32  *     it is now executing code in unloaded module, so it crashes
  33  *
  34  * Note that if the process that takes the foreign reference to dm kobject
  35  * has a low priority and the system is sufficiently loaded with
  36  * higher-priority processes that prevent the low-priority process from
  37  * being scheduled long enough, this bug may really happen.
  38  *
  39  * In order to fix this module unload race, we place the release method
  40  * into a helper code that is compiled directly into the kernel.
  41  */
  42
  43 void dm_kobject_release(struct kobject *kobj)
  44 {
  45         complete(dm_get_completion_from_kobject(kobj));
  46 }
  47
  48 EXPORT_SYMBOL(dm_kobject_release);