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Date: Mon, 5 Dec 2016 11:46:52 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Dmitry Vyukov <dvyukov@google.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>
Subject: Re: Unkillable processes due to PTRACE_TRACEME again
Message-ID: <20161205104652.GA29197@redhat.com>
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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.

Oleg.