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Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 30 Aug 2021 07:32:07 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Zhang Qiao <zhangqiao22@huawei.com>, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group
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References: <20210826112635.7404-1-zhangqiao22@huawei.com>
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Hello,

On Mon, Aug 30, 2021 at 04:39:54PM +0200, Peter Zijlstra wrote:
> > When a new process is forked, cgroup_post_fork() associates it
> > with the cgroup of its parent. Therefore this commit move the
> > __set_task_cpu() and task_fork() that access some cgroup-related
> > fields(sched_task_group and cfs_rq) to sched_post_fork() and
> > call sched_post_fork() after cgroup_post_fork().

I think this would allow cgroup migrations to take place before
sched_post_fork() is run, which likely will break stuff. The right
thing to do likely is taking sched_task_group (and whatever other
fields) after cgroup_can_fork(), which fixates the cgroup memberships,
is run. For other controllers, operations like this would be performed
from cgroup_subsys->fork() callback but it's tricky for sched due to
autogroup.

> > Fixes: 8323f26ce342 ("sched: Fix race in task_group")
> > Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> 
> Hmm, I think you're right. Did something recently chagne in cgroup land
> to make this more visible? This code hasn't changed in like 9 years.

I can't think of any remotely recent change either. I guess ppl just
don't try to migrate the parent while fork is in progress.

Thanks.

-- 
tejun