Re: [PATCH V6 01/10] Use copy_process in vhost layer

Mike Christie
2022-02-02T21:03:05Z

On 1/18/22 1:12 PM, Eric W. Biederman wrote:
> Mike Christie <michael.christie@oracle.com> writes:
>
>> On 1/17/22 11:31 AM, Eric W. Biederman wrote:
>>> Mike Christie <michael.christie@oracle.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.
>
> Bah good point. We won't send the signal but you won't autoreap either.
>
> I think we could legitimately change this bit:
>
> /*
> * Send with __send_signal as si_pid and si_uid are in the
> * parent's namespaces.
> */
> if (valid_signal(sig) && sig)
> __send_signal(sig, &info, tsk->parent, PIDTYPE_TGID, false);
>
> To add:
> else
> /* We don't notify the parent so just autoreap */
> autoreap = true;
>

Hey,

This works for me, but I think we might have issues where threads now get
reaped too soon when they are being ptraced.

I think I found a simple solution by just using kernel_wait in the vhost
task code since I want to wait for the thread to exit when I'm removing
a device already. I posted a patchset so you can check it out.