linux-kernel - Re: [PATCH v3.4-rc5] block: iocontext->nr

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Date:	Tue, 01 May 2012 14:31:07 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Tejun Heo <tj@...nel.org>, Dave Jones <davej@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [PATCH v3.4-rc5] block: iocontext->nr_tasks should be initialized to one

Jens Axboe <jaxboe@...ionio.com> writes:

> On 2012-05-01 20:09, Tejun Heo wrote:
>> On Tue, May 01, 2012 at 08:02:39PM +0200, Jens Axboe wrote:
>>> On 2012-05-01 18:17, Tejun Heo wrote:
>>>> create_task_io_context() left ioc->nr_tasks at zero; however, a newly
>>>> created ioc should have its nr_tasks initialized to one as it begins
>>>> attached to the task creating it.
>>>>
>>>> This affects only CLONE_IO which currently doesn't seem to have any
>>>> actual user.  Sasha triggered WARN_ON_ONCE() in ioc_task_link() using
>>>> syscall fuzzer.  Even when it happens, the failure mode isn't critical
>>>> (blk-cgroup may allow attaching a CLONE_IO'd task to a cgroup when it
>>>> shouldn't and blkcg limits may behave weirdly).
>>>
>>> CLONE_IO is an exported interface, it can be set from clone(2).
>>> Otherwise Sasha would not have hit this :-)
>> 
>> Yeah, but with pthread not exposing it, I'm very skeptical how much,
>> if any, use it's getting.  With its incompatibility with blk-cgroup
>> and cfq being able to merge coop request streams, I'm not sure how
>> much we need it.  Maybe we can just make it noop?
>
> It's a lot more robust and specific than hoping to get coop merging. For
> cfq, it also implies that multiple threads sharing an io context should
> be accounted as one.
>
> But as to actual users, I really don't know. I agree it's probably not
> that widely used. If google still had that code search, we could get a
> better idea :-)

I know of one project: the venerable dump/restore utility uses CLONE_IO.

Cheers,
Jeff
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