linux-kernel - Re: [REGRESSION] [PATCH] block: uninitialized ioc->nr_tasks triggers WARN

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Date:	Tue, 24 Jul 2012 11:18:58 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"Maxim V. Patlasov" <mpatlasov@...allels.com>
Cc:	Olof Johansson <olof@...om.net>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [REGRESSION] [PATCH] block: uninitialized ioc->nr_tasks
 triggers WARN_ON

Hello,

On Tue, Jul 24, 2012 at 05:35:30PM +0400, Maxim V. Patlasov wrote:
> 07/18/2012 02:24 AM, Tejun Heo пишет:
> >Hello, Olof.
> >
> >On Tue, Jul 17, 2012 at 02:36:43PM -0700, Olof Johansson wrote:
> >>It seems like the init of ioc->nr_tasks was removed in that patch,
> >>so it starts out at 0 instead of 1.
> >>
> >>Tejun, is the right thing here to add back the init, or should something else
> >>be done?
> >>
> >>The below patch removes the warning, but I haven't done any more extensive
> >>testing on it.
> >>
> >>
> >>Signed-off-by: Olof Johansson <olof@...om.net>
> >Right, the patch shouldn't have replaced the init.
> >
> >  Acked-by: Tejun Heo <tj@...nel.org>
> >
> >Fortunately, the effect of the bug is limited.  ioc->nr_tasks only
> >used to veto block cgroup migration if a task has ioc which is shared
> >by multiple tasks.  Currently, the only known program using CLONE_IO
> >is dump and even if somebody migrates some threads of a single dump
> >instance to a different block cgroup, the result won't be catastrophic
> >although block cgroup policies would become ambiguous.  IMHO, it
> >should be okay to route this through -stable after 3.5.  Jens?
> 
> Please notice that annoying WARN_ON comes from world-visible
> ioc_task_link(). So any third-party module using ioc_task_link()
> ends up in that clutter in logs. E.g. OpenVZ ploop block-device uses
> ioc_task_link().

This should go through block tree.  Jens, ping.

Thanks.

-- 
tejun
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