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Date:   Fri, 25 Feb 2022 18:37:35 +0100
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Vipin Sharma <vipinsh@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Tejun Heo <tj@kernel.org>,
        seanjc@google.com, lizefan.x@bytedance.com, hannes@cmpxchg.org,
        dmatlack@google.com, jiangshanlai@gmail.com, kvm@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] KVM: Move VM's worker kthreads back to the original
 cgroups before exiting.
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Hi Vipin.

On Wed, Feb 16, 2022 at 09:37:45AM -0800, Vipin Sharma <vipinsh@google.com> wrote:
> On Thu, Jan 20, 2022 at 7:05 AM Michal Koutn? <mkoutny@suse.com> wrote:
> > Have I missed an obstacle?

Aha...

[
> I used few other combination where I put kernel_wait() call after
> put_task_struct(k) call.
> 
> Every time during the module exit, kernel was crashing like:

Thanks for trying this out.
]

> Do you have any suggestions what might be the right way to use this API?

...it has occured to me now -- the KVM kthread is not a child of the
wanna-wait user task. So the kernel_wait() silently errs with -ECHILD
and task_struct is released too early and that (probably) brings about
the crash.

I'm sorry for not realizing that initially.

(Generally, any kthread_create'd task would be affected by this. I guess
the KVM worker threads can't be forked from the kvm_create_vm() callers?
(It could prevent the double migration to and from caller's cgroup
though.))

Michal