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


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