Messages in this thread | | | Date | Sun, 28 Apr 2019 18:24:06 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v1 1/2] Add polling support to pidfd |
| |
Thanks for cc'ing me...
On 04/26, Christian Brauner wrote: > > On Thu, Apr 25, 2019 at 03:00:09PM -0400, Joel Fernandes (Google) wrote: > > +static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts) > > +{ > > + struct task_struct *task; > > + struct pid *pid; > > + int poll_flags = 0; > > + > > + /* > > + * 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); > > + pid = file->private_data; > > + task = pid_task(pid, PIDTYPE_PID); > > + WARN_ON_ONCE(task && !thread_group_leader(task)); > > + > > + if (!task || (task->exit_state && thread_group_empty(task))) > > + poll_flags = POLLIN | POLLRDNORM;
Joel, I still can't understand why do we need tasklist... and I don't really understand the comment. The code looks as if you are trying to avoid poll_wait(), but this would be strange.
OK, why can't pidfd_poll() do
poll_wait(file, &pid->wait_pidfd, pts);
rcu_read_lock(); task = pid_task(pid, PIDTYPE_PID); if (!task || task->exit_state && thread_group_empty(task)) poll_flags = POLLIN | ...; rcu_read_unlock();
return poll_flags;
?
> > +static void do_notify_pidfd(struct task_struct *task) > > Maybe a short command that this helper can only be called when we know > that task is a thread-group leader wouldn't hurt so there's no confusion > later.
Not really. If the task is traced, do_notify_parent() (and thus do_notify_pidfd()) can be called to notify the debugger even if the task is not a leader and/or if it is not the last thread. The latter means a spurious wakeup for pidfd_poll().
> > +{ > > + struct pid *pid; > > + > > + lockdep_assert_held(&tasklist_lock); > > + > > + pid = get_task_pid(task, PIDTYPE_PID); > > + wake_up_all(&pid->wait_pidfd); > > + put_pid(pid);
Why get/put?
Oleg.
| |