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SubjectRe: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd
On Fri, Mar 13, 2015 at 12:57 PM,  <josh@joshtriplett.org> wrote:
> On Fri, Mar 13, 2015 at 05:21:13PM +0100, Oleg Nesterov wrote:
>> Josh,
>>
>> I'll certainly try to read this series, but not before next week.
>
> Thanks for looking at it.
>
>> but a couple of nits right now.
>>
>> On 03/12, Josh Triplett wrote:
>> >
>> > When passed CLONE_FD, clone4 will return a file descriptor rather than a
>> > PID. When the child process exits, it gets automatically reaped,
>>
>> And even I have no idea what you are actually doing, this doesn't look
>> right, see below.
>>
>> > +static unsigned int clonefd_poll(struct file *file, poll_table *wait)
>> > +{
>> > + struct task_struct *p = file->private_data;
>> > + poll_wait(file, &p->clonefd_wqh, wait);
>> > + return p->exit_state == EXIT_DEAD ? (POLLIN | POLLRDNORM) : 0;
>> > +}
>> > +
>> > +static ssize_t clonefd_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>> > +{
>> > + struct task_struct *p = file->private_data;
>> > + int ret = 0;
>> > +
>> > + /* EOF after first read */
>> > + if (*ppos)
>> > + return 0;
>> > +
>> > + if (file->f_flags & O_NONBLOCK)
>> > + ret = -EAGAIN;
>> > + else
>> > + ret = wait_event_interruptible(p->clonefd_wqh, p->exit_state == EXIT_DEAD);
>> > +
>> > + if (p->exit_state == EXIT_DEAD) {
>>
>> Again, I simply do not know what this code does at all. But I bet the usage
>> of EXIT_DEAD is wrong ;)
>>
>> OK, OK, I can be wrong. But I simply do not see what protects this task_struct
>> if it is EXIT_DEAD (in fact even if it is EXIT_ZOMBIE).
>
> If by "what protects" you mean "what keeps it alive", the file
> descriptor holds a reference to the task_struct by calling
> get_task_struct when created and put_task_struct when released.
>
> This wait_event_interruptible pairs with the wake_up_all called from
> clonefd_do_notify, which exit_notify calls *after* setting the task to
> TASK_DEAD.
>
> Apart from that, what about what the code is doing isn't clear?
>
>> > @@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>> > if (group_dead)
>> > kill_orphaned_pgrp(tsk->group_leader, NULL);
>> >
>> > - if (unlikely(tsk->ptrace)) {
>> > + if (tsk->autoreap) {
>> > + autoreap = true;
>>
>> Debuggers won't be happy. A ptraced task should not autoreap itself.
>
> A process launching a new process with CLONE_FD is explicitly requesting
> that the process be automatically reaped without any other process
> having to wait on it. The task needs to not become a zombie, because
> otherwise, it'll show up in waitpid(-1, ...) calls in the parent
> process, which would break the ability to use this to completely
> encapsulate process management within a library and not interfere with
> the parent's process handling via SIGCHLD and wait{pid,3,4}.

Wouldn't the correct behavior be to keep it alive as a zombie but
*not* show it in waitpid, etc?

--Andy


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