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

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Date:	Tue, 24 Jul 2012 17:35:30 +0400
From:	"Maxim V. Patlasov" <mpatlasov@...allels.com>
To:	Tejun Heo <tj@...nel.org>
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

Hi Tejun,

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().

Thanks,
Maxim

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