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
43void dm_kobject_release(struct kobject *kobj)
44{
45	complete(dm_get_completion_from_kobject(kobj));
46}
47
48EXPORT_SYMBOL(dm_kobject_release);
49