linux-kernel - Re: [PATCH] cgroup/cpuset: Fix a race condition when reading cpuset.*

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Message-ID: <20200212221543.GL80993@mtj.thefacebook.com>
Date:   Wed, 12 Feb 2020 17:15:43 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup/cpuset: Fix a race condition when reading cpuset.*

On Tue, Feb 11, 2020 at 02:15:54PM +0000, Qais Yousef wrote:
> LTP cpuset_hotplug_test.sh was failing with the following error message
> 
> 	cpuset_hotplug 1 TFAIL: root group's cpus isn't expected(Result: 0-5, Expect: 0,2-5).
> 
> Which is due to a race condition between cpu hotplug operation and
> reading cpuset.cpus file.
> 
> When a cpu is onlined/offlined, cpuset schedules a workqueue to sync its
> internal data structures with the new values. If a read happens during
> this window, the user will read a stale value, hence triggering the
> failure above.
> 
> To fix the issue make sure cpuset_wait_for_hotplug() is called before
> allowing any value to be read, hence forcing the synchronization to
> happen before the read.
> 
> I ran 500 iterations with this fix applied with no failure triggered.
> 
> Signed-off-by: Qais Yousef <qais.yousef@....com>

Hello, Qais. I just applied a patch which makes the operation
synchronous. Can you see whether the problem is gone on the
cgroup/for-next branch?

  git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next

Thanks.

-- 
tejun