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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 11 Apr 2020 11:29:04 -0700
Message-ID: <CAHk-=wgwXpKepChGi4ZhQVxZxD0ic8s2CDXvUmqBTMaKGz-fjg@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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On Sat, Apr 11, 2020 at 11:21 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 04/09, Linus Torvalds wrote:
> >
> >  (1) have execve() not wait for dead threads while holding the cred
> > mutex
>
> This is what I tried to do 3 years ago, see

Well, you did it differently - by moving the "wait for dead threads"
logic to after releasing the lock.

My simpler patch was lazier - just don't wait for dead threads at all,
since they are dead and not interesting.

Because even if it's Easter weekend, those threads are not coming back
to life ;)

You do say in that old patch that we can't just share the signal
state, but I wonder how true that is. Sharing it with a TASK_ZOMBIE
doesn't seem all that problematic to me. The only thing that can do is
getting reaped by a later wait.

That said, I actually am starting to think that maybe execve() should
just try to reap those threads instead, and avoid the whole issue that
way. Basically my "option (2)" thing.

Sure, that's basically stealing them from the parent, but 'execve()'
really is special wrt threads, and the parent still has the execve()
thread itself. And it's not so different from SIGKILL, which also
forcibly breaks off any ptracer etc without anybody being able to say
anything about it.

                Linus