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From: xuejiufei <jiufei.xue@linux.alibaba.com>
To: Tejun Heo <tj@kernel.org>, Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	Caspar Zhang <caspar@linux.alibaba.com>,
	linux-block <linux-block@vger.kernel.org>,
	cgroups@vger.kernel.org
Subject: Re: [PATCH v2] blk-throttle: fix race between blkcg_bio_issue_check and cgroup_rmdir
Date: Fri, 23 Feb 2018 09:56:54 +0800	[thread overview]
Message-ID: <bc6304d1-5b76-c867-e76a-ab1c87b604ee@linux.alibaba.com> (raw)
In-Reply-To: <20180222151721.GA1641506@devbig577.frc2.facebook.com>

Hi Tejun,

On 2018/2/22 下午11:18, Tejun Heo wrote:
> Hello,
> 
> On Thu, Feb 22, 2018 at 02:14:34PM +0800, Joseph Qi wrote:
>> I still don't get how css_tryget can work here.
>>
>> The race happens when:
>> 1) writeback kworker has found the blkg with rcu;
>> 2) blkcg is during offlining and blkg_destroy() has already been called.
>> Then, writeback kworker will take queue lock and access the blkg with
>> refcount 0.
> 
> Yeah, then tryget would fail and it should go through the root.
> 
In this race, the refcount of blkg becomes zero and is destroyed.
However css may still have refcount, and css_tryget can return success
before other callers put the refcount.
So I don't get how css_tryget can fix this race? Or I wonder if we can
add another function blkg_tryget?

Thanks,
Jiufei

> Thanks.
> 

  reply	other threads:[~2018-02-23  1:56 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-07  8:40 [PATCH v2] blk-throttle: fix race between blkcg_bio_issue_check and cgroup_rmdir Joseph Qi
2018-02-07 21:38 ` Tejun Heo
2018-02-08  2:29   ` Joseph Qi
2018-02-08 15:23     ` Tejun Heo
2018-02-09  2:15       ` Joseph Qi
2018-02-12 17:11         ` Tejun Heo
2018-02-22  6:14           ` Joseph Qi
2018-02-22 15:18             ` Tejun Heo
2018-02-23  1:56               ` xuejiufei [this message]
2018-02-23 14:23                 ` Tejun Heo
2018-02-24  1:45                   ` Joseph Qi
2018-02-27  3:18                     ` Joseph Qi
2018-02-27 18:33                     ` Tejun Heo
2018-02-28  6:52                       ` Joseph Qi
2018-03-04 20:23                         ` Tejun Heo
2018-03-05  1:17                           ` Joseph Qi

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