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

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Message-ID: <20120309212049.GD24890@google.com>
Date:	Fri, 9 Mar 2012 13:20:49 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mikulas Patocka <mpatocka@...hat.com>,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	Alasdair G Kergon <agk@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Elly Jones <ellyjones@...omium.org>,
	Milan Broz <mbroz@...hat.com>,
	Olof Johansson <olofj@...omium.org>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: workqueues and percpu (was: [PATCH] dm: remake of the verity
 target)

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
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