linux-kernel - Re: [PATCH v2] kernel/sched: Fix sched_fork() access an invalid sched_task

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Date:   Wed, 15 Sep 2021 11:10:42 +0800
From:   Zhang Qiao <zhangqiao22@...wei.com>
To:     Tejun Heo <tj@...nel.org>
CC:     <mingo@...hat.com>, <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>, <juri.lelli@...hat.com>,
        <vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
        <rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
        <bristot@...hat.com>
Subject: Re: [PATCH v2] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group

在 2021/9/15 0:23, Tejun Heo 写道:
> On Tue, Sep 14, 2021 at 04:20:02PM +0800, Zhang Qiao wrote:
>> 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().
> 
> Can't we just add a can_fork callback in cpu_cgrp_subsys?

Because __set_task_cpu and task_fork callback need access the sched_task_group field, so the task_fork
callback must be called after update the child's sched_task_group, if perform these
operations in the newly added can_fork callback, it may lead to some dependencies(the can_fork will
not be executed when !CONFIG_CGROUP_SCHED).

__set_task_cpu and task_fork callback mainly do some field initialization and vruntime calculation,
these things just need to be done before waking up the new task. So i think it is no problem to defer
do this things at sched_post_fork().

thanks.

>