linux-kernel - Re: [PATCH V6 01/10] Use copy

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Date:   Tue, 18 Jan 2022 13:00:39 -0600
From:   Mike Christie <michael.christie@...cle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     geert@...ux-m68k.org, vverma@...italocean.com, hdanton@...a.com,
        hch@...radead.org, stefanha@...hat.com, jasowang@...hat.com,
        mst@...hat.com, sgarzare@...hat.com,
        virtualization@...ts.linux-foundation.org,
        christian.brauner@...ntu.com, axboe@...nel.dk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 01/10] Use copy_process in vhost layer

On 1/18/22 12:51 PM, Mike Christie wrote:
> On 1/17/22 11:31 AM, Eric W. Biederman wrote:
>> Mike Christie <michael.christie@...cle.com> writes:
>>
>>> On 12/22/21 12:24 PM, Eric W. Biederman wrote:
>>>> All I am certain of is that you need to set
>>>> "args->exit_signal = -1;".  This prevents having to play games with
>>>> do_notify_parent.
>>>
>>> Hi Eric,
>>>
>>> I have all your review comments handled except this one. It's looking like it's
>>> more difficult than just setting the exit_signal=-1, so I wanted to check that
>>> I understood you.
>>
>> [snip problems with exit_signal = -1]
>>
>>>
>>> What do you think?
>>
>> I was wrong.  I appear to have confused the thread and the non-thread
>> cases.
>>
>> Perhaps I meant "args->exit_signal = 0".  That looks like
>> do_notify_parent won't send it, and thread_group_leader continues to do
>> the right thing.
> 
> That doesn't work too. exit_notify will call do_notify_parent but 
> our parent, qemu, does not ignore SIGCHILD so we will not drop
> down in into this chunk:
> 
>         psig = tsk->parent->sighand;
>         spin_lock_irqsave(&psig->siglock, flags);
>         if (!tsk->ptrace && sig == SIGCHLD &&
>             (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
>              (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
> 
> do_notify_parent will return false and so autoreap in exit_notify will
> be false.
> 
> 
> 
>>
>> Baring any additional confusion on my part that cleanly solves the
>> problem of how not to send a signal from a child process cleanly.
>>


Oh yeah, maybe we are thinking about different things.

The issue I am hitting is that the parent, qemu, does not know about
these task_structs. And, userspace can add/delete these vhost devices
dynamically. So qemu could continue to run, but the vhost device could
be deleted. In this case I want to free up the task_struct resources
since we are no longer using it.

With kthreads do_notify_parent returns true and exit_notify calls
release_task, so it's handled for me. I had stuck in the USER/VHOST
check in the patch you didn't like to get that behavior.

If you prefer not adding the VHOST/USER task check in exit_notify then
instead of auto reaping, would another possible alternative be to add a
modified kernel_wait like function in the vhost layer to reap the task?