Re: [dm-devel] workqueues and percpu (was: [PATCH] dm: remake of the verity target)

[Tejun Heo <tj kernel org>]

Hello,

On Fri, Mar 09, 2012 at 01:15:12PM -0800, Mandeep Singh Baines wrote:
> In this case, I need the per-cpu data for the duration of calculating
> a cryptographics hash on a 4K page of data. That's a long time to disable
> pre-emption.

How long are we talking about?  Tens of microsecs, tens of millisecs?

> I could fix the bug temporarily by adding get/put for the per_cpu data
> but would that be acceptable? I'm not sure what the OK limit is for how
> long one can disable preemption. An alternative fix would be not allow
> CONFIG_VERITY when CONFIG_HOTPLUG_CPU. Once workqueues are fixed, I could
> remove that restriction.

I think the right thing to do for now is to add cpu hotplug notifier
and do flush_work_sync() on the work item.  We can later move that
logic into workqueue and remove it from crypto.

Thanks.

-- 
tejun