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SubjectRe: [PATCH RFC 1/2] Add polling support to pidfd
On 04/11, Joel Fernandes (Google) wrote:
>
> +static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
> +{
> + int poll_flags = 0;
> + struct task_struct *task;
> + struct pid *pid;
> +
> + task = get_proc_task(file->f_path.dentry->d_inode);
> +
> + WARN_ON_ONCE(task && !thread_group_leader(task));
> +
> + /*
> + * tasklist_lock must be held because to avoid racing with
> + * changes in exit_state and wake up. Basically to avoid:
> + *
> + * P0: read exit_state = 0
> + * P1: write exit_state = EXIT_DEAD
> + * P1: Do a wake up - wq is empty, so do nothing
> + * P0: Queue for polling - wait forever.
> + */
> + read_lock(&tasklist_lock);
> + if (!task)
> + poll_flags = POLLIN | POLLRDNORM | POLLERR;
> + else if (task->exit_state == EXIT_DEAD)
> + poll_flags = POLLIN | POLLRDNORM;
> + else if (task->exit_state == EXIT_ZOMBIE && thread_group_empty(task))
> + poll_flags = POLLIN | POLLRDNORM;
> +
> + if (!poll_flags) {
> + pid = proc_pid(file->f_path.dentry->d_inode);
> + poll_wait(file, &pid->wait_pidfd, pts);
> + }

can't understand...

Could you explain when it should return POLLIN? When the whole process exits?

Then all you need is

!task || task->exit_state && thread_group_empty(task)

Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably
live in task->signal_struct.

Oleg.

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