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

[Mike Galbraith]

On Tue, 2017-05-09 at 08:53 -0600, Jens Axboe wrote:
> On 05/09/2017 12:07 AM, Mike Galbraith wrote:
> > 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
> 
> Is it from this_cpu_ptr() in blk_stat_add()?

Yeah.

> > ..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.
> 
> That part is fine. The might sleep depends on BLOCKING being set in
> the hardware queue flags. And if that is set, then we don't call
> it under get_cpu().

Ah, right.

	-Mike