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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 7 Sep 2021 07:01:06 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Zhang Qiao <zhangqiao22@huawei.com>
Cc:     peterz@infradead.org, juri.lelli@redhat.com,
        linux-kernel@vger.kernel.org, mingo@redhat.com,
        vincent.guittot@linaro.org
Subject: Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group
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Hello,

On Thu, Sep 02, 2021 at 03:42:15PM +0800, Zhang Qiao wrote:
> I checked the code again.
> I don't quite understand what you said, if the child be moved between
> cgroup_post_fork() and sched_post_sched(), what problems might it cause?

cgroup_post_fork() is where the child's creation is committed from cgroup's
POV, so it'd be migrating cgroups before the initial creation is finished.
From glancing, looks like it'll break css_set task counts to begin with.
This violates the basic assumptions and can cause critical failures in
subtle ways. The would replace one subtle race with a possibly worse one.

Thanks.

-- 
tejun