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Subject: Re: [PATCH v2] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group
To:     Tejun Heo <tj@kernel.org>
CC:     <mingo@redhat.com>, <peterz@infradead.org>,
        <linux-kernel@vger.kernel.org>, <juri.lelli@redhat.com>,
        <vincent.guittot@linaro.org>, <dietmar.eggemann@arm.com>,
        <rostedt@goodmis.org>, <bsegall@google.com>, <mgorman@suse.de>,
        <bristot@redhat.com>
References: <20210911075054.6358-1-zhangqiao22@huawei.com>
 <YT+R3EnFgs78Vyvh@slm.duckdns.org>
From:   Zhang Qiao <zhangqiao22@huawei.com>
In-Reply-To: <YT+R3EnFgs78Vyvh@slm.duckdns.org>
Message-ID: <b30205b0-bf54-7255-fc1c-9dfa5dbb4940@huawei.com>
Date:   Tue, 14 Sep 2021 16:20:02 +0800
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?? 2021/9/14 2:01, Tejun Heo д??:
> Hello,
> 
> On Sat, Sep 11, 2021 at 03:50:54PM +0800, Zhang Qiao wrote:
>> Between cgroup_can_fork() and cgroup_post_fork(), the cgroup
>> membership is fixed and thus sched_task_group can't change. So
>> call sched_fork() after cgroup_can_fork() and update the child's
>> sched_task_group before it is used.
> 
> The part being fixed looks correct to me but it's difficult to for me to
> assess whether the whole relocation of the sched_fork() hook doesn't change

Indeed is this, although I checked and tested it many times.

> anything else. Besides, even if we decide to relocate the sched_fork hook, I
> think it'd be better to separate the two changes - one is a relatively safe
> bug fix, the other is a code reorganization with possibly subtle side
> effects. So, I think it'd be better to produce a patch which just fixes the
> bug even if that ends up introducing another function in the flow.

Perhaps we can fix this bug at sched_post_fork(), which is executed between cgroup_can_fork()
and cgroup_post_fork(). Relocate the fectching of task_group and access it at sched_post_fork().

thanks.

> 
> Thanks.
>