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From: Dmitry Vyukov <dvyukov@google.com>
Date: Mon, 5 Dec 2016 15:07:03 +0100
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Subject: Re: Unkillable processes due to PTRACE_TRACEME again
To: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Machek <pavel@ucw.cz>, Denys Vlasenko <dvlasenk@redhat.com>,
        jan.kratochvil@redhat.com, palves@redhat.com,
        Roland McGrath <roland@hack.frob.com>,
        syzkaller <syzkaller@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Dec 5, 2016 at 12:00 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 12/05, Oleg Nesterov wrote:
>>
>> On 12/02, Dmitry Vyukov wrote:
>> >
>> > I am not on 2caceb3294a78c389b462e7e236a4e744a53a474 (Dec 1). And see
>> > the same unwaitable zombie processes.
>>
>> This is another thing, and notabug. This is how ptrace works,
>>
>> > void *thr(void *arg)
>> > {
>> >   ptrace(PTRACE_TRACEME, 0, 0, 0);
>> > }
>> >
>> > int main()
>> > {
>> >   int pid = fork();
>> >   if (pid == 0) {
>> >     pthread_t th;
>> >     pthread_create(&th, 0, thr, 0);
>> >     usleep(100000);
>> >     exit(0);
>> >   }
>> >   usleep(200000);
>> >   kill(pid, SIGKILL);
>> >   int status = 0;
>> >   waitpid(pid, &status, __WALL);
>>
>> waitpid(pid) hangs because you need to reap the sub-thread first.
>
> I'm afraid I wasn't clear...
>
> So the child process has 2 threads, the leader thread L and the sub-thread T.
> waitpid(pid == L->pid) will block until all the threads go away, but since T is
> traced it won't autoreap, the tracer should do waitpid(T->pid) first to reap
> this zombie. waitpid(-1) should work too.

Do you mean that I need to replace:
 waitpid(pid, &status, __WALL);
with:
 while (waitpid(-1, &status, __WALL) != pid) {}
?
It seems to work. But want to make sure.