On Tue, Oct 20, 2015 at 10:34 AM, Dmitry Vyukov <dvyu...@google.com> wrote:
> On Mon, Oct 19, 2015 at 10:17 PM, Dmitry Vyukov <dvyu...@google.com> wrote:
>> On Mon, Oct 19, 2015 at 9:49 PM, Oleg Nesterov <o...@redhat.com> wrote:
>>> On 10/19, Dmitry Vyukov wrote:
>>>>
>>>> The following program hangs in some interesting state and is not
>>>> killable (started by a normal user, not root):
>>>
>>> Thanks.
>>>
>>>> #include <pthread.h>
>>>> #include <unistd.h>
>>>> #include <sys/ptrace.h>
>>>> #include <stdio.h>
>>>> #include <signal.h>
>>>>
>>>> void *thr(void *arg) {
>>>> ptrace(PTRACE_TRACEME, 0, 0, 0);
>>>> sleep(3);
>>>> kill(getpid(), SIGCHLD);
>>>> return 0;
>>>> }
>>>>
>>>> int main() {
>>>> if (fork() == 0) {
>>>> sleep(1);
>>>> pthread_t th;
>>>> pthread_create(&th, 0, thr, 0);
>>>> sleep(1);
>>>> }
>>>> return 0;
>>>> }
>>>>
>>>>
>>>> The child process attaches as tracee to init process
>>>
>>> Yes, although in a racy manner, the parent can exit after
>>> PTRACE_TRACEME in this case the kernel will untrace the task
>>> before reparenting. Not that this matters.
>>>
>>>> and then hangs in
>>>> a state that I don't understand. When I did a similar thing but
>>>> attached it to a normal parent process (shell), I still was able to
>>>> get rid of it by killing parent (shell).
>>>
>>> See above.
>>>
>>> So I bet the problem is that your /sbin/init doesn't use __WALL,
>>> so wait() doesn't reap the traced zombie sub-thread, and thus it
>>> can't release the non-empty thread group.
>>>
>>> Could you please verify? Just do "strace -p1" and send SIGCHLD to
>>> init.
>>>
>>> perhaps eligible_child() should assume WALL if ptrace && ZOMBIE...
>>
>>
>> I am using Ubuntu.
>> Here strace output from init:
>>
>> waitid(P_ALL, 0, {}, WNOHANG|WEXITED|WSTOPPED|WCONTINUED, NULL) = 0
>>
>> So what should be fixed here? Kernel of distro init?
>
> waitpid(__WALL) indeed joins these processes.
> But __WALL can't be used with waitid and Ubuntu init uses waitid...
I am thinking how to workaround this issue.
The following program joins both child processes:
#include <pthread.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <stdio.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
void *thr(void *arg) {
ptrace(PTRACE_TRACEME, 0, 0, 0);
return 0;
}
int main() {
int pid = fork();
if (pid == 0) {
pthread_t th;
pthread_create(&th, 0, thr, 0);
sleep(1);
return 0;
}
siginfo_t info = {};
int status = 0;
int res = waitpid(-1, &status, __WALL);
printf("pid=%d res=%d errno=%d\n", pid, res, errno);
res = waitpid(-1, &status, __WALL);
printf("pid=%d res=%d errno=%d\n", pid, res, errno);
return 0;
}
However, I need to wait for a particular child and if I change the
first waitpid to:
int res = waitpid(pid, &status, __WALL);
then it does not terminate.
So how can I wait for such child process?
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