On Tue, Apr 16, 2019 at 02:04:31PM +0200, Oleg Nesterov wrote:
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?
It returns POLLIN when the task is dead or doesn't exist anymore, or when it is in a zombie state and there's no other thread in the thread group.
Then all you need is
!task || task->exit_state && thread_group_empty(task)
Yes this works as well, all the tests pass with your suggestion so I'll change it to that. Although I will the be giving up returing EPOLLERR if the task_struct doesn't exit. We don't need that, but I thought it was cool to return it anyway.
Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably live in task->signal_struct.
About wait_pidfd living in signal_struct, that wont work since the waitqueue has to survive for the duration of the poll system call. Linus also confirmed this: https://lore.kernel.org/patchwork/patch/1060650/#1257371
Also the waitqueue living in struct pid solves the de_thread() issue I mentioned later in the following thread and in the commit message: https://lore.kernel.org/patchwork/comment/1257175/
thanks,
- Joel