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SubjectRe: [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.

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