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

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Date:	Tue, 1 May 2012 20:18:22 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.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

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 :-)

-- 
Jens Axboe

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