dm crypt: fix cpu hotplug crash by removing per-cpu structure

dm crypt: fix cpu hotplug crash by removing per-cpu structure

The DM crypt target used per-cpu structures to hold pointers to a
ablkcipher_request structure.  The code assumed that the work item keeps
executing on a single CPU, so it didn't use synchronization when
accessing this structure.

If a CPU is disabled by writing 0 to /sys/devices/system/cpu/cpu*/online,
the work item could be moved to another CPU.  This causes dm-crypt
crashes, like the following, because the code starts using an incorrect
ablkcipher_request:

 smpboot: CPU 7 is now offline
 BUG: unable to handle kernel NULL pointer dereference at 0000000000000130
 IP: [<ffffffffa1862b3d>] crypt_convert+0x12d/0x3c0 [dm_crypt]
 ...
 Call Trace:
  [<ffffffffa1864415>] ? kcryptd_crypt+0x305/0x470 [dm_crypt]
  [<ffffffff81062060>] ? finish_task_switch+0x40/0xc0
  [<ffffffff81052a28>] ? process_one_work+0x168/0x470
  [<ffffffff8105366b>] ? worker_thread+0x10b/0x390
  [<ffffffff81053560>] ? manage_workers.isra.26+0x290/0x290
  [<ffffffff81058d9f>] ? kthread+0xaf/0xc0
  [<ffffffff81058cf0>] ? kthread_create_on_node+0x120/0x120
  [<ffffffff813464ac>] ? ret_from_fork+0x7c/0xb0
  [<ffffffff81058cf0>] ? kthread_create_on_node+0x120/0x120

Fix this bug by removing the per-cpu definition.  The structure
ablkcipher_request is accessed via a pointer from convert_context.
Consequently, if the work item is rescheduled to a different CPU, the
thread still uses the same ablkcipher_request.

This change may undermine performance improvements intended by commit
c0297721 ("dm crypt: scale to multiple cpus") on select hardware.  In
practice no performance difference was observed on recent hardware.  But
regardless, correctness is more important than performance.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org