Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

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:

quoted

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.

quoted

+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?

quoted

@@ -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}.

- Josh Triplett