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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
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Date:   Mon, 06 Jun 2022 17:10:02 -0500
In-Reply-To: <20220525142845.GA2687@redhat.com> (Oleg Nesterov's message of
        "Wed, 25 May 2022 16:28:46 +0200")
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Subject: Re: [PATCH 07/16] signal: Wake up the designated parent
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Oleg Nesterov <oleg@redhat.com> writes:

> On 05/24, Oleg Nesterov wrote:
>>
>> On 05/24, Oleg Nesterov wrote:
>> >
>> > I fail to understand this patch...
>> >
>> > On 05/18, Eric W. Biederman wrote:
>> > >
>> > > Today if a process is ptraced only the ptracer will ever be woken up in
>> > > wait
>> >
>> > and why is this wrong?
>> >
>> > > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()")
>> >
>> > how does this change fix 75b95953a569?
>>
>> OK, I guess you mean the 2nd do_notify_parent_cldstop() in ptrace_stop(),
>> the problematic case is current->ptrace == T. Right?
>>
>> I dislike this patch anyway, but let me think more about it.
>
> OK, now that I understand the problem, the patch doesn't look bad to me,
> although I'd ask to make the changelog more clear.

I will see what I can do.

> After this change __wake_up_parent() can't accept any "parent" from
> p->parent thread group, but all callers look fine except
> ptrace_detach().

Having looked at it a little more I think the change was too
restrictive.  For the !ptrace_reparented case there are possibly
two threads of the parent process that wait_consider_task will
allow to wait even with __WNOTHREAD specified.  It is desirable
to wake them both up.

Which if I have had enough sleep reduces this patch to just:

diff --git a/kernel/exit.c b/kernel/exit.c
index f072959fcab7..c8156366b722 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1431,8 +1431,10 @@ static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode,
        if (!eligible_pid(wo, p))
                return 0;
 
-       if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent)
-               return 0;
+       if ((wo->wo_flags & __WNOTHREAD) &&
+           (wait->private != p->parent) &&
+           (wait->private != p->real_parent))
+                       return 0;
 
        return default_wake_function(wait, mode, sync, key);
 }


I think that solves the issue without missing wake-ups without adding
any more.

For the same set of reasons it looks like the __wake_up_parent in
__ptrace_detach is just simply dead code.  I don't think there is a case
where when !ptrace_reparented the thread that is the real_parent can
sleep in do_wait when the thread that was calling ptrace could not.

That needs a very close look to confirm. 

Eric