get/put_cpu() usage in block/blk-mq.c

[Mike Galbraith]

Hi Jens,

I was about to fix up this splat..

[  445.022141] loop: module loaded
[  445.078116] BUG: using smp_processor_id() in preemptible [00000000] code: loop0/3801
[  445.085873] caller is debug_smp_processor_id+0x17/0x20
[  445.091016] CPU: 7 PID: 3801 Comm: loop0 Tainted: G            E   4.12.0-default #40
[  445.098838] Hardware name: IBM System x3550 M3 -[7944K3G]-/69Y5698     , BIOS -[D6E150AUS-1.10]- 12/15/2010
[  445.108564] Call Trace:
[  445.111016]  dump_stack+0x65/0x89
[  445.114330]  check_preemption_disabled+0xde/0xf0
[  445.118945]  debug_smp_processor_id+0x17/0x20
[  445.123300]  blk_stat_add+0xb0/0x130
[  445.126876]  __blk_mq_complete_request+0xb5/0x150
[  445.131575]  blk_mq_complete_request+0x16/0x20
[  445.136020]  loop_queue_work+0x5f/0xaa0 [loop]
[  445.140461]  ? _raw_spin_unlock_irq+0x21/0x40
[  445.144816]  ? finish_task_switch+0x85/0x270
[  445.149085]  ? __schedule+0x291/0x8c0
[  445.152747]  kthread_worker_fn+0xc2/0x1d0
[  445.156754]  kthread+0x114/0x150
[  445.159983]  ? __kthread_init_worker+0xb0/0xb0
[  445.164424]  ? kthread_park+0x60/0x60
[  445.168085]  ret_from_fork+0x2c/0x40

..but then noticed that __blk_mq_run_hw_queue() is also called under
get_cpu(), but recently grew a might_sleep(), so maybe wants some block
layer eyeballs.

	-Mike