A NULL pointer is seen if two concurrent "vgchange -ay -K <vg name>"
processes race to load the dm-thin-pool module:

 PID: 25992 TASK: ffff883cd7d23500 CPU: 4 COMMAND: "vgchange"
  #0 [ffff883cd743d600] machine_kexec at ffffffff81038fa9
  0000001 [ffff883cd743d660] crash_kexec at ffffffff810c5992
  0000002 [ffff883cd743d730] oops_end at ffffffff81515c90
  0000003 [ffff883cd743d760] no_context at ffffffff81049f1b
  0000004 [ffff883cd743d7b0] __bad_area_nosemaphore at ffffffff8104a1a5
  0000005 [ffff883cd743d800] bad_area at ffffffff8104a2ce
  0000006 [ffff883cd743d830] __do_page_fault at ffffffff8104aa6f
  0000007 [ffff883cd743d950] do_page_fault at ffffffff81517bae
  0000008 [ffff883cd743d980] page_fault at ffffffff81514f95
     [exception RIP: kmem_cache_alloc+108]
     RIP: ffffffff8116ef3c RSP: ffff883cd743da38 RFLAGS: 00010046
     RAX: 0000000000000004 RBX: ffffffff81121b90 RCX: ffff881bf1e78cc0
     RDX: 0000000000000000 RSI: 00000000000000d0 RDI: 0000000000000000
     RBP: ffff883cd743da68 R8: ffff881bf1a4eb00 R9: 0000000080042000
     R10: 0000000000002000 R11: 0000000000000000 R12: 00000000000000d0
     R13: 0000000000000000 R14: 00000000000000d0 R15: 0000000000000246
     ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
  0000009 [ffff883cd743da70] mempool_alloc_slab at ffffffff81121ba5
 0000010 [ffff883cd743da80] mempool_create_node at ffffffff81122083
 0000011 [ffff883cd743dad0] mempool_create at ffffffff811220f4
 0000012 [ffff883cd743dae0] pool_ctr at ffffffffa08de049 [dm_thin_pool]
 0000013 [ffff883cd743dbd0] dm_table_add_target at ffffffffa0005f2f [dm_mod]
 0000014 [ffff883cd743dc30] table_load at ffffffffa0008ba9 [dm_mod]
 0000015 [ffff883cd743dc90] ctl_ioctl at ffffffffa0009dc4 [dm_mod]

The race results in a NULL pointer because:

Process A (vgchange -ay -K):
 	a. send DM_LIST_VERSIONS_CMD ioctl;
 	b. pool_target not registered;
 	c. modprobe dm_thin_pool and wait until end.

Process B (vgchange -ay -K):
 	a. send DM_LIST_VERSIONS_CMD ioctl;
 	b. pool_target registered;
 	c. table_load->dm_table_add_target->pool_ctr;
 	d. _new_mapping_cache is NULL and panic.
Note:
 	1. process A and process B are two concurrent processes.
 	2. pool_target can be detected by process B but
 	_new_mapping_cache initialization has not ended.

To fix dm-thin-pool, and other targets (cache, multipath, and snapshot)
with the same problem, simply dm_register_target() after all resources
created during module init (as labelled with __init) are finished.

Cc: stable@vger.kernel.org
Signed-off-by: monty <monty_pavel@sina.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>